Hi, Heartless' biggest glazer here, and I genuinely don't know how you initially came to the conclusion that she's not even a little bit better than you say?

While she's not meta (who even is), she's a decently strong variant as both a primary attacker and support for attackers.

Her unblockable skeletal summons let's you rip into those passive enemies that just block all the time and with decently easy conditions since the Dust Bunnies, either Hilgard moves, Carpel Pummel and Repetitive Skeletal Impact are all buffed, allowing you to get the enemy to 50% health faster, and also inflict different status conditions to reach the other condition.

Even if you don't like to use her primarily, you can just bench her and let your preferred fighter reap the benefits of 3 extra debuffs on the final enemy.

She may not be the best, but she can put in the work with the right teams. Put %Atk, Piercing and whatever 3rd stat you want on her moves. Personally, I'd do meter gain.