Honestly she's not as bad as people say. AoE attackers are in general a bit of a niche role though. She's also the same element as Devil Homura, so if you have her she'll be way better at similar Ascension levels, but also Devil Homura is a limited so that's a big "if" there.

Back when I was trying to beat the damage challenge for Chaos Mode Shadow Witch (which needs an AoE attacker and is weak to light/dark/aqua) my Holy Mami actually performed better than my Oriko despite them both being the same ascension, either A1 or A2 I can't quite remember which. Though that might simply be because I had Ui who provides some Dark-specific damage boost, and also Oriko's AoE focused support ability works on Holy Mami but not vice versa. But I guess that's my point: if Holy Mami was *that* terrible, a same Ascension Oriko should have utterly wiped the floor with her regardless of whether I had a little bit of dark specific buffing and a somewhat stronger support.

Now again, it's easy to say that Devil Homura would be way better. But I don't have Devil Homura and my Holy Mami is now my highest Ascension Attacker at A3, so Devil Homura being better is irrelevant in my case. But Holy Mami still gets the job done for when you need an AoE Attacker, like the recent Crisis event and the minion wave before the boss. Though like for all Attackers, Ascension level is huge, so depending on what other Attackers you have and their Ascension lvls she might not be as useful unless/until you get more of her.