The Mutation Situation!

Mutations occur when someone's biology breaks and fractures out of existence and promptly gets overwritten by new genetic code that — more often than not — is permanent and irreversible; this all occurs in a single extremely painful and traumatizing instant before said pain fades like nothing happened.
They're more likely to naturally manifest in one's preteens, but it's not unheard of for them to appear in young children or adults alike.

They come in all shapes and sizes: they may be small and insignificant (maybe your eyes are a slightly different hue), wildly beneficial (you're suddenly more charismatic), or harmful in every sense of the word (gaining a chronic never-before-seen disorder) and nearly always come in bunches — it's exceedingly rare to have only one change.
…However, we're gonna blow right past this because I'm already verbose enough as-is and because mutated individuals are far more well-known for having superpowers and a lot of them being supers.

Nonsensical Nonsense

Mutations are complete and utter randomness bestowed upon random people without rhyme nor reason. No one knows how or why. Even worse, it's difficult to examine because the genetic code of mutated people is (often described as) 'scrambled' or 'nightmarish to look at,' constantly shifting and changing as though it were a sentient encryption.

…However, weirdly enough, there lies some semblance of 'order' within this randomness as shown through phenotypes.

Magical Non-Magic?

Due to the bullshit nature of mutations, literally anything could be a considered a superpower. Anything at all as long as it's superhuman in nature, whether that be wings protruding out of your back or super laser piss.
Although, while there are abilities that bend humanity's understanding of physics, the natural world, and other laws of reality, there haven't been any abilities that are downright incomprehensible. Not yet, anyway.
Life is a result of the Universe's mind fracturing literally beyond belief. Anything within the human consciousness technically resides within the Universe's by proxy; think of a game of broken telepathic Telephone.
Superpowers are manifested by drawing from, twisting, and breaking what already exists, kind of like drawing names from a hat.

HOWEVER, abilities must be at least somewhat based on the natural world. There aren't any magical girls with mysterious magical glitter emanating from their magical heart wands. No, sir.
There also exists a limit to how many abilities one can have before their body begins to deteriorate from the strain of both mutating so many times in one lifespan and maintaining so many superpowers at once. The exact limit is a case-by-case basis that depends on who is receiving said abilities (because everyone's body is different) and what those powers are. You can assume that thirty powers or more means game over though.

Usual mutations usually involve a random 'theme' of abilities* along with some or all of what's been colloquially referred to as the (Super) Starter Pack (TM): superhuman strength, senses, agility, durability, endurance, reflexes, and/or equilibrium.
Each of these powers come with their own byproducts: someone with superhuman strength may have higher metabolism to be able to maintain said strength, a person whose abilities are rabbit-like in nature may develop paranoia, someone with superior endurance may also have better stamina in bed… Things like that.

Theoretical & Actual Might

The stronger the ability relative to others, the less likely it is to manifest, especially if it requires little-to-no training to wield. There's no one with god powers running around. The infinite possibilities of superpowers make it difficult to definitively 'rank' them, but if you put two people who share an ability against each other, one of the two will surely have a 'better' manifestation of it than their opponent.

Theoretical might is how well an ability might fare in general (usually in reference to combat, but not exclusive), while actual might is how it scores against other people who share that same ability. Is it subjective and confusing? Yup. What can ya do.

Anyway, for example, super speed has surprisingly low theoretical might compared to other abilities due to how easy it is to circumvent: they could be teleported or moved via telekinesis, their legs could be broken (and pulling a handstand is gonna make them much slower), what-have-you. However, they could outrun most people who also have super speed, making their actual might fairly high.
Speaking of mind stuff, telekinesis has a high theoretical might due to its wide range of uses: you could chuck something heavy at your opponent, lift a lot of people or things at once, lift yourself out of a dangerous situation, etc. Although, what if it's been barely trained, making someone only be able to lift light objects? Then, their actual might would be low compared to other telekinetics.

Mutate-a-Thon

There's a slim — but nonzero — chance of a mutated individual mutating again, whether that be an abrupt 'course correction' to remove one's abilities or 're-mutating,' gaining new abilities to either replace the old ones or be added on top of them. This is performed via the same method of initially altering someone's genetic structure of the whole reality-breaking thing.
More often than not, these additional mutations add bonus abilities matching the same 'theme' of the previous ones. For example, someone whose powers are draconic in nature may gain horns or an unexplainable need to hoard things.

Spot the Difference

Mutations present differently in every featherless biped, but identifying humans, mutates, and mutants is determined by two factors:

If this occurs naturally without any sort of external stimuli, such individuals would be referred to as mutants — Homo superior.
Other times, this happens in response to an event that causes abrupt mutagenic change, including but not limited to: severe emotional distress, 'contagion' from another mutated being, mental episodes, etc., which would classify an individual as a mutate — Homo medialis.
People who don't mutate ever in their entire lives are your regular Homo sapiens.

Anti-Mutant Views

Among mutated communities, there's been talk of discrimination and negative views from non-mutated humans regarding mutations and mutated people. Sure, it's a bit ironic considering how much society loves supers, but the fear and wariness surrounding mutations isn't completely absent.

Humans have never been trusting of the unknown nor those different than them. In fact, when mutated people began popping up more and more, devout folk had assumed a sort of Doomsday — or some other event hailing the End of All Things — was transpiring. Therefore, mutated individuals were treated with suspicion and wariness if not outright distaste. Either that, or they were vehemently worshipped due to being seen as divinity of some kind. There was very little in-between.

However, most modern dislike stems from fear and desperation. Not because of the whole 'otherness' thing or even evolutionary superiority, surprisingly, but because people are terrified of being subject to people's dangerous abilities; what if their mind is secretly being read 24/7 or their neighbor randomly self-destructs out of rage, taking the whole neighborhood with them?
Not to mention the constant, looming fear of being caught between crossfire and there's nothing you can do. The ever-present worry of becoming bargaining chips of bad actors looking to exploit the abilities of mutated people or someone they know. The horror of a known mutated individual taking advantage of non-mutated people just because they're stronger, faster, and smarter. The stress of losing employment opportunities and recognition due to some new hire with superpowers being able to complete the job of ten men with the salary of one. The ease and flippancy of a mutated individual obliterating the lives of countless people at any given time with nothing more than a thought…

The list of concerns surrounding mutated people seems to be endless, and the sheer amount of sensationalism surrounding the superpowered scene and superhuman affairs certainly fails to assuage such worries. All of the stories within that sphere are so absolutely absurd, that they all begin to blend together, resulting in people harboring disdain for mutated people entirely.