If you were an Earthbender, and defeated Slade, would the Titans let you be a member? That depends. Do you look like Terra? If so, maybe not; that association carries a lot of baggage. Are you a basically nice person, or a pain in the butt? Robin won't take new members who would cause friction (Jinx is an exception, but those are special circumstances). On balance, barring "special" causes, I would think YES. Earthbending is a very useful talent.