And Kevin Smith's right, completely. Superman is the straightest, most normal hero there has ever been... He's a serious character and is supposed to be taken seriously.
Deadpool on the other hand isn't, continuity, plausible weapons aren't a part of him (I'm only thinking about early Deadpool as well - never mind the latest shit).
TF2 is more on the Deadpool side of things; I mean it started with weapons that didn't technically make sense (teleporter, healing beam etc..) and now it's expanding on those fundamentals, naturally it's getting sillier as it goes along but it's all in name of gameplay really.
Mann co is a bit like the crazy aperture science - but set in a cartoony universe so it can afford to be a bit silly.
The thing is, though, I still don't think things like a wrench making you more flammable even make
cartoon sense. There's a difference between something silly and impossible but still within the confines of even convoluted cartoon logic - like a healing beam or a teleporter - and something that's not just silly and impossible but a total non sequitur.
I'm not trying to get in an argument, but I really don't see TF2 as being fundamentally as off the wall as you suggest. Teleporters, Mediguns, etc. are impossible, sure, but they make sense as long as you accept that "they have magic solutions to combat problems," i.e. a magic healing beam that seals up bullet wounds and pads you stand on to instantly transport you to the front lines.
Things like a 10% vulnerability to fire damage from a wrench with spikes on it goes beyond the initial willing suspension of disbelief. The reason I compared it to Superman is that Superman's ability to fly is the core of his character and is necessary for all things that make him unique (fighting Brainiac on the ground? Lame), but his amnesia-kissing was just made up on the spot to fill a potential plothole.
These goofy stat-changes are just that: the heal beam of the Medigun is necessary for the Medic to even exist, and all it amounts to is just "okay, it's just ridiculously high tech", but +10% fire vulnerability, -20 HP, 65% faster weapon switch, etc. are just random stat changes slapped on to balance out the other gameplay mechanics those weapons have.