ReVo says beleta, but I've never liked to use -et in compliments. It sounds kind of like "not good enough to be beautiful".
Dolĉa is an option. It corresponds to the German "süß". But can it be used for sweet appearance, or only for sweet taste?
Another one I've seen is ĉarma. Does this word have anything to do with "charming"? Or is it closer to "cute"?