Is it bad to put a sticker on the back of your phone?

We would have no way to answer that question since we don't have any idea what the sticker you put on has for adhesive. Most stickers leave a residue when you remove them. You are going to have to apply your own common sense to answer your question. Only you can make the determination regarding the prudence of putting a sticker on the back of your device. If you do, just make sure you don't cover up any connections, cameras, speakers, etc.

Best,

GB

Again, we can't make that call for you. No one here is a sticker expert. You need to make the decision.

GB

Depends on the sticker. You can put the sticker on a cheap iPhone case.