I mean...I don't really have a problem with it unless a person does ZERO research on the fandom. I mean, I agree that people can and should cosplay as whoever they want to. However, I think that when people cosplay from something they aren't familiar with...it kind of defeats the purpose. I mean, if a person does no research on the character or series, how will he/she pose for photos while remaining in-character? And how will he/she interact with fans of the series? I would just hate for someone to get bashed by fans for not knowing anything about the series at least.
I mean, most people that ask for your picture are fans of the character or the series that the character is from. And to not pose in-character for the person would be a little disheartening to that person. I mean, personally when I take a picture I love when the cosplayer poses in character. It looks awesome! And then there are times where the person will just stand there and I'm like....oh
ok then... Of course, then again some people will take your picture because they like the outfit ^^
Overall though, I'm about 95% ok with people cosplaying as characters from a fandom they aren't too familiar with. Personally, I try to avoid it, but if I do choose a character to go with a friend or because of the design, I'll force myself to play/watch the series. I usually end up enjoying it regardless though, so I actually use cosplay to get involved with new series. The only thing I ask people is that they do a little bit of research at least so they are familiar with the character and the series ^^ It will lead to a more pleasant experience when interacting with fans and posing for pictures.