This was covered somewhat in my suggestion for mutation flavour text, but I thought it’d be helpful to separate the two.
The basic idea is that gaining a mutation is over a period of time after mutagen intake, with symptoms manifesting due to the bodily changes (feathers/spines pushing through the skin, night vision causing pain and temporary blindness in sunlight during development, etc.) much like a regular disease.
This would also allow, perhaps, for something akin to ‘prevention’: if your character suddenly feels horrible when they go outside (but not enough to affect stats) you’re gonna drink some purifier, which might auto-cure developing mutations with priority over ‘settled’ mutations.
The effects would likely manifest perhaps a few hours to a couple days after consuming the mutagen, and be almost universally negative (some exceptions might include developing traits like Slimy, which would likely just increase wet benefits for a while and leave no trail while you’re covered in only a thin layer of mucus).
That’s the gist of the idea. I am aware I’ve brought this up already.