The reason Rose's character development (and Greg's, to a certain extent) happens in reverse and makes those characters look worse at the end of the story than at the beginning is because SU was always about "growing up" rather than anything else and part of growing up is realizing your parents are shitty cunts most of the time and you didn't realize it until you got old enough to have some adult perspective and realize all the times they fucked up hard in ways you consider to be unacceptably retarded but got away with it because you were too young to call them on their bullshit.
Was pink a good mom? I don't think that's the point of the story. I think the entire point of the story is that your family can be shitty, terrible people but you'll always be stuck with their shitty, terrible decisions and maybe even be kind of shitty and terrible yourself (steven's depression turning him into pink/tyrannical steven and from there into a monster).
The only message that can be 100% taken from the story is that even if you're separate by a million miles and several lifetimes or don't even know them (Steven's grandparents) your family will always remain your family, for good or ill. Everything else in the story is merely meant to frame that story or keep you involved long enough to witness the Universe family drama play out.
This was, probably, the overall message Sugar wanted to tell since it's really the only thing you can meaningfully take from the story.