The problem was she took up way too much of the final season. Her role should have been much smaller, and she shouldn't have turned into a generic Hollywood "strong woman" after becoming good either.
Ideally her purpose should have been to cement Jack's attachment to the world of the present, to make his final choice of leaving to the past feel more consequential. You could even keep the whole her being Aku-Jr shit unchanged. For example, in the final battle Ashy realizes she can create time portals like before. However, she cannot pass through them herself, since Aku has never shown being able to do this either. She uses her power in a heartfelt goodbye to send Jack to the past, where he gets a final showdown with past Aku, while she stays in the present to try to fix it on her own. That way both the past and present plotlines get satisfying endings, instead of un-creating the whole present timeline and letting everyone there die.