What makes a good burner username
A good throwaway username reveals nothing about you: not your real name, birth year, city or other handles you use. Keeping burner identities separate from your real one is the whole point of using a disposable account.
Combine a random username with a temporary email address and a one-off password and you have a complete burner identity for one-time sign-ups, free trials or testing, with no link back to you.