Mero-mero is an expression of being infatuated or in love. Pon is from ponkotsu, which means clumsy in a cute way, so it's just meant to be a cutesy suffix. Dayu I think, is just a wordplay on -dayo, which is a more casual and cutesy form of -desu. So Meropon-dayu is meant to be an overly cutesy name that makes everyone grossed out to see a nasty man like astel using it for an online name 😂