I never personally ran into any problems with the fandoms I am in outside of online, but people tend be act obnoxious online due to the anonymity behind it or don't see it as a big deal as in isn't in person (that that can said about online in general).
Offline, I hadn't really met other fans due to the area I am in and how small some of the fandoms are and most of the ones I met online had been nice so far (well at least to me anyways).
I'm sure every fandom, no matter how big or small, will have those obnoxious fans that stands out as well as those nice and or more mature/well behaved fans.
Though I don't really see how skill level of others cosplayers or how certain costumes that are more worn really make fans/fandom annoying. Not everyone is a professional or have a lot of time, money or know how to sew (some might just be new to cosplay or the series and putting them down might discourage them. Isn't the point really to have fun and show your love? Not every enters competitions) and though it might be nice to see other costumes, some people just like certain characters or outfits more.