Like.. Mrs Banana? Mrs Orange? Something funny I guess..
A good username would be something that best describes you. Like your personality....
Something simple. No numbers (Numbers can be confusing), and something people can easily remember. Like, for example, mine is Tilted Halo. (Not saying that mine is the only good username, it's just an example.) But you can change your username any time, as long as you have 20 Gems.
I like Down to Earth names, mine's Lost Without You, But I'm changing mine.