People about Cosplay...
Honestly it’s all about what you personally find to work best, but for me I put on the costume first, or at least part of it.
For starters, it makes sure that the makeup doesn’t stain the neck hole of your costume, if there is one. This way all you have to worry about is usually powder fallout and that can be cleaned easily. You could also cover part of your chest with a towel like a bib if you’re really worried about stains.
If you’re putting on a wig it also helps to not dirty that or have it in the way of your work.
Also I find that the makeup tends to look better with the pieces around it. Makeup that is over the top or has unnatural aspects will sometimes look really silly with your normal hair and clothes, so to save the worry that you’re not doing it right, it helps to have the rest on. Especially when the wig comes on it all comes into place and you can do needed touch ups.
I often will only put on half the costume though. For example if the costume involves a jacket, belt, jewelry, etc that I can put on without touching the makeup, I’ll wait until the end. This also helps the makeup since costume pieces might not be made with full mobility in mind and can weigh you down or prevent movement.
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.
To me, it’s equally a tribute to the property that I’m making the cosplay from, and the art form.
One could say the same thing of not getting the appeal of creating any media – learning any instrument, painting a picture, writing a book. It’s creation and display – this may not be the same to everyone, but it’s definitely a big part to me. It’s also a way to meet new friends from all walks of life. I’m certain if you looked closer, a lot don’t meet the stereotypes of an overgrown kid. I cosplay with people in the military, aerospace, tech, and all walks of life. Conversely there’s people that do this for a living, myself included.
As someone that enjoys hobbies and media that was formerly shunned for being “too nerdy”, perhaps immediately throwing anyone that takes part in cosplay into the “you’re a man/woman child” box is a bit reductionist, and a little offensive.