You use 'in appearance' - the pronoun 'their' is not required, and usually omitted. 'On appearance' is simply the wrong preposition, that's all.
Also, "Bodybuilders are usually looked brawny..." is ungrammatical. It must either be 'Bodybuilders ARE usually brawny...' or 'Bodybuilders usually LOOK brawny...'. The combination 'are...looked...' is not possible.