People about Cosplay...
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.
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.
There’s a few things – one is that you you’re going to get asked for pictures, especially if you cosplay a popular character.
For conventions, make sure that you have a way of carrying essentials on you, even if your costume doesn’t have pockets. Perhaps make a simple bag to match your costume (same color as your outfit, or decorated with a suitable design, an example being a Jedi costume with the Jedi order symbol).
Wear your costume around before the convention, to make sure that you can move around in it, or that things don’t start slipping/pinching/falling apart after a little while. At home, there’s lots of time to make adjustments and fixes.
Don’t worry if you’re not as good as some of the veteran or professional cosplayers out there. Those cosplayers started somewhere too, and with practice, you too will eventually get good.