No, topless is not common at the ruins or any busy tourist beach.
Public nudity is illegal in Mexico.

Sure, you can walk from the ruins to the beach, because they are adjacent. Obvious? Very.

I might get in trouble with this description of my impression of the way the rule is enforced but here goes. Angelina Jolie types can walk around topless all day. Rosie O'Donnell might get told to cover up.
Sorry for the over simplification, but that is how it seems to work.