The MPAA officially lets you get away with one F-bomb (or other "harsher sexually-derived swear") in a PG-13 movie, as long as it is not used in a sexual context but rather as a simple expletive, and may let you off with two if the movie is otherwise mostly or just about devoid of other objectionable content (violence, drug use, sexuality, etc.). At least one movie (As Good as It Gets) has managed to slip by with three F-bombs and no violence/drugs/sex. Either way you'd need to think very carefully about the timing if you want to have swearing but restrict yourself to that number.
Swearing is very much a characterization thing; for some characters it feels right that they do, and for others it would feel out of place. It's also a mood/atmosphere thing, though; a character who would swear in a grittier story would probably feel out of place doing so in, say, an otherwise lighthearted action/adventure story with kid main characters. It generally feels forced if it seems like the author is only including swearing in order to seem cool and edgy, instead of it being a meaningful aspect of characterization: if, for example, everyone just seems to pepper their words with swearing in the same way, regardless of who they are or whether the situation calls for it.
Personally, swearing doesn't put me off at all unless it's forced like that; usually it's just a neutral thing. Though really well-timed precision F-strikes can actively add to a scene, and it can be an additional dash of flavor to the characterization. I've personally written both fics with no swearing and where it would just be inappropriate to the mood, a couple with one or two F-bombs coming from characters in a very agitated state, and one where one of the characters swears almost constantly when he's stressed, angry or upset, some of the others swear every now and then, and others don't at all. It all depends on what works for the story and characters.
~Butterfree/Dragonfree/antialiasis of
The Cave of Dragonflies
Still not going to sprite for your fangame. Sorry, but I don't really sprite or give out permission for people to use my fake Pokémon anymore.