Mama's dream

It seems to me that her violent/cruel behavior is the result of some mental illness or severe personality disorder... probably an extreme eating disorder

To put it simply: she is crazy

The question is... why? which is the reason for her insanity?

Probably eating too much sweets clouding her moral judgement?

I think she was an abandoned child for her to want a family so much. Not to mention she wants to be family with all the races and had a picnic without a care in the world so it means she's not really interested to be a pirate king nor she wants "One Piece".