I can understand her in a way. She gives birth to a beautiful baby boy and the first thing all mothers want to do is hug their new baby, but of course as soon as she does he turns in a rabbit. Imagine if you will not the cute bunnie that Momiji is, but if it were real life it would just be a fluffy rabbit that munches carrots!! Imagine how horrified she must have been when that happened for the first time.
It's not as though she didn't try either. Momiji was a small child before her memory was erased and I guess those years of trying to cope were just too much for her to handle.

If you think back to all the mothers, they each react differently; Momiji's tries but can't cope. Kisa's overcompensates and almost smothers Kisa. Yuki's is a bit of a tyrant and wants him to do as he's told and makes decisions for him (she wants to ingraciate herself with Akito) etc. I don't think we know much about the others mothers (or at least not in the anime and not up to vol. 13 of the manga). It's just human nature I guess.