People about Cosplay...
The only place it should really matter is in competition. Don’t enter a workmanship based contest with something that you bought or commissioned, and don’t take credit for the work of others.
That said, some contests have a category for commissioned or purchased costumes, you would be fine if you entered there.
Putting aside for the moment that the tone of the question doesn’t imply the most open of minds, cosplay is – simply – acting. Do you consider Mel Gibson, Ken Branagh, David Tennant, Cumberbatch, et. al. immature for wanting to dress up and act as Hamlet? Cosplay is a similar – if not identical – mindset: people, inspired by characters, possessing the right combination of talent, bravery and self-awareness, performing as said character for an audience. Whether it’s a convention stage or a Broadway stage makes no difference. The person who portrays Deadpool or Master Chief is very much spiritual kin to the person who plays King Lear or Hamlet. It’s wrong to praise one and denigrate the other based on the source of their inspiration, for to declare which works are “great” and which are not is subjective or worse. Still, even if most consider it blasphemy to compare Marvel to Shakespeare, the motivation to portray characters from either realm is identical and the healthy mindset WON’T criticize this, realizing that whatever it doesn’t “get” is not automatically inferior or invalid.
PS: Ironically you will find, if you but ask, that most cosplayers as children were very successful Trick or Treaters, as Halloween is quite often the “bug” that bites them for their subsequent motivation.
First of all, cosplay isn’t just about playing dress-up. Cosplay is a whole lot more than that. Good cosplayers will act like the character while in costume or move like the character. Serious cosplayers won’t break character when they’re in costume. It’s a little bit of acting too. But it’s a culture that’s meant to be fun.
You see, cosplay is a couple things. First, it’s a service to the fandom because other people get to essentially see their favorite characters in the flesh for a couple hours. And this is especially true if you really fit the character. But it’s a lot more than that. It’s a chance to be someone else for a little while. When you put on cosplay, you get to leave your life behind for a little while. And for the people who make their own costumes, it’s a chance to not only show off their knowledge of the character, but it allows them to show off their costume skills and say “I made that.”
So, there are a lot of different appeals to doing cosplay. Besides, with all the photos that people take of you in a day, it kind of makes you feel like a celebrity. :)