In fact, being a bad parent is the one reliable symptom of villainy on OUAT. Similarly, villains are made sympathetic via their “correct(ed)” attitude to motherhood/fatherhood (hence Rumple, Zelena, Regina, Maleficient, and eventually Cora all have their sympathetic side revolve around their relationship to their children, and their redemption, when applicable, is framed around that too).
This is a valid point. I have always had a deep problem with this aspect of the show. Women especially seem to have their worth measured through their parenthood. It’s archaic and slightly misogynistic.
It’s that the show systematically equates ambivalence, rejection, or improper interpretation of parenthood (see Cora & BF) with moral bankruptcy. I think we can discuss whether that’s a problematic message or not, but I think that’s what’s behind the audience’s dislike of Milah.
The show presents many ideas to the audience – whether the audience accepts them is another question. To use a contentious example, the show’s been telling us that CS is a great romance for years – but as we all know, there is a faction in the fandom that strongly reject that idea. There are many other examples as well. However, the case of Milah is interesting because the audience has swallowed the show’s predominant line about her without any protest at all.