People about Cosplay...
Absolutely! Just not in the way your potentially-leading question could be interpreted to imply.
Wikipedia says that “cosplay” broadly applies to any costumed role-playing in venues apart from the stage. Therefore, it’s only the growing and disturbing layman’s trend of equating “cosplay” with “manga/Japanese” which makes it appear there’s a problem. My generation was cosplaying before the term was coined (c.1984), and thus we “costumed” at conventions as characters from Star Trek, Star Wars, Ghostbusters, Doctor Who, Marvel and DC comics, etc., none of which are anime or in any other way Japanese in culture. Even if some were, most other characters equally originated from different countries and cultures, so on the surface the answer would appear to be “no”.
Now, is cosplay appropriation of any kind? It’s actually nothing but! By definition, cosplay is of a previously-established character from a previously-established distributed media venue (TV, film, books, etc.). You’re always cosplaying a character which someone else invented, or at least basing in on one (crossplayers, etc.). You’re not asking the creator of said character to do so, so yes, you’re appropriating it. But it’s an appropriation of trademark or license issues, not of culture… all cultures are potentially fair game for it.
Finally, cosplay itself has matured and spread enough that Wikipedia already classifies it as a “subculture”, which I suspect is a conservative definition if you’ve ever been to a comic-con.
So: cosplay is, in the end, a “culture” based on “appropriation”. If we concede that this encompasses any culture and appropriation, then your answer is a resounding and unabashed “yes”.
No, it is perfectly normal to buy a cosplay costume. Even professional cosplayers buy or get the costume made from someone else. However, making your own costume has a different thrill to it.
What is important though, is how you carry the costume and how much fun you’re having while cosplaying.
As for “devaluing someone’s participation” goes, some may feel that your cosplay is superior to them, when they took more efforts. However, it is not supposed to be that way as these things are done for fun.
To answer your main questions, No and No. Cosplay is supposed to be about inclusiveness, fun, and showing your love for a character.
That being said, as with any group of people, there will be jerks on both sides. There will be those that make their costumes and look down on those that buy them, and vice versa. Jerks can be found at all levels of cosplay, just like regular society. And just like regular society, you have the choice to ignore the jerks and go your own way.
If you are happy with your cosplay, not competing in a 100% bought cosplay and are having fun, that really is all that matters.
People who make their cosplays receive just as many nasty comments and critiques as those that buy them by the way.