People about Cosplay...
Absolutely not! There are exceptions, but even seasoned cosplayers who enjoy the crafting side of things will often buy parts of their costume. There’s an entire segment of the community that does commission work who wouldn’t have any business at all without the parts of the community who find skipping to the dressing up stage to be most fulfilling.
So long as you are having a good time and participating ethically (not taking credit for a costume someone else made, for example), you are doing cosplay exactly right!
It’s a way of expressing your love for a particular anime/manga/video game series. You enjoy the world the author created so much that you feel like dressing up as one of the characters. It’s a tribute of sorts.
It’s fun to take a break from your real life and escape into an anime/manga/video game world, even if just for a day at an anime convention. You get to be someone else for a bit.
Great cosplayers draw crowds of convention-goers, get their picture taken by lots of photographers and receive lots of compliments. It’s flattering.
When you walk through a convention and see other cosplayers, there’s a sense of belonging to a community of people with shared interests. People make eye contact, smile when they recognize your character, ask to take pictures and are generally pretty friendly.
Some people really enjoy the process of planning and creating the costume. Making armor, sewing, and building weapons can be a fun and rewarding hobby.
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.