Would it really be this easy to steal peoples bitcoins? I was just looking for an Ethereum wallet app for Android, and something hit me. 

Could someone create a wallet app for Android/iOS that simply pretends to create a wallet address but really just displays the address of the one who created the app, causing the victim to transfer his/her funds straight to the app creator instead of their (supposedly) newly created wallet?

This seems too easy...
Yes, of course this would work.

What do you expect? It's the digital equivalent of walking into a random building with a storefront that vaguely looks like a bank with lots of dollar signs, and then asking "hey i want a bank account, where do I send money?".

Don't install random wallet software. Do research.