Can a crap parent make a good grandparent?

Yes. A lot of really authoritarian parents change their ways as they become older. This phenonemon is actually very peculiar & it can work both ways. For example: My Grandad and my Grandmother. As parents, my grandad was the horrible parent. He was never at home, he got angry easily, he hit his kids and yelled at them for minor infractions. My grandmother was the passive, stress free- easygoing one. Flip foward and now I live with my grandmother and now she's stressful, authoritive (but not a bad parent). My Grandad who I don't live with but see very often, is now very passive and likeable. Apparantly he is the antithesis of everything he was thirty years ago. It's weird how people change so drastically.

its possible mostly good parents make even better grandparents though

My parents were really strict with us, although I don't blame them because there was seven children and it can't have been easy for them. They mellowed when I had my children and like you say, the certainly over indulged them. Sometimes, I used to think it was to make up for the way we were rared. I am now a grandmother myself and I love spoiling my grandchildren, but isn't that what grandparents are for.

I think that's often true. When they were parents, they may have had money problems, marriage problems, maybe they were still really immature themselves and just didn't know how to parent. By the time they become grandparents they've ironed through their own issues, they're more secure in their lives and of course they only have the grandchild occasionally. It's not their responsibility to feed, clothe, shelter & love this child- they only have to love this child. That's a lot easier to provide when you don't have to do the rest.