Actually it's similar to "Apalah aku ini?" which means "I am nobody". someone say "aku mah apa atuh?" is trying to say that he/she is nobody. I hope it helps :D

Phrase "aku mah apa atuh ?" is an expression usually used to tell that "I'm nothing if i compared with ..." , it's hard to explain it. Phrase "aku rapopo" comes from Javanese, originally written as "aku ora opo-opo", it means "I'm Fine".

yeah but I still don't understand the meaning of this expression

This isn't really pure Indonesian, it mixed with the Javanese (rapopo) and Sundanese (aku mah apa atuh). Often happened here :D