First off, just because a species is exposed to a mutagen does not always mean it'll turn out the same as before. FEV only does that because it's hard-coded to do that. Radiation, on the other hand, is a far different thing in and of itself. The odds that the same mutation will pop up twice is pretty low. Occam's Razor would imply that all giant mantises come from the same place.
Of course, Bethesda kinda screwed that up with all those returning monsters, so... Yeah.
Also, may I once again add, IIRC wasn't there, like, a great big black rain that killed off most of the plants? Who knows, maybe I'm thinking of something else.
1)
Mutation also doesn't meant that a creature's biology is infinitely moldable and plastic. The mutation twiddles the wrong thing and you simply die or become very ill. It's not odd to think that "getting bigger" is biologically very plausible and easy under Fallout rules of biology or that similar species would have similar "viable" mutations that wouldn't kill or sterilize it. Which seems to be the general case. Scorpions get bigger. Bighorn sheep get bigger. Mantises get bigger. You get the idea.
Also: Don't lecture me on the razor if you have no idea what the principle of parsimony actually is. "Simple" doesn't mean whatever you arbitrarily or subjectively decide is "simple." It means that you violate the fewest accepted facts and evidence and have do not use ad hoc hypothesis. It's also an informal rule of logic, not an ironclad law.
Either way, I believe I expressed that I don't really care anymore
how the mantises got big because it's a handwave to allow mantises in Vault 22, although I don't think it's clear what the scientists there
actually did. It wouldn't be odd if they irradiated the dang things for whatever reason in addition to working on a fungal pesticide. Mutation breeding with radiation is a
thing in the real world. If it's a common practice among Pre-War scientists, then it probably isn't surprising if even enlarging the species is just a side product of whatever else they wanted to actually study.
2)
There are Clarke entries that mentions that stuff survived in Zion in spite of radioactive precipitation, albeit either with harmless but nonfunctional mutations or by being drastically altered by it. And he specifically names the exact species that you find surviving in the game.
Basic survival of the fittest. What's to get. The radiation dropped off in a couple months and while that did kill a lot of things, a number of other species survived for whatever reason, whether by pure blind luck or some other adaptable trait they possessed.
Personally, ravens I can well believe. The damn things are fucking intelligent and are pervasive enough to be pests.