When I was 16 my mom signed over her parental rights, I am now 23 and have a kid. What rights to her grand kid does she have?

She has any grandparent rights that you allow her or if she takes it to court then whatever the Judge says but that would probably be just visitations. But since she signed away rights a Judge wouldn't give her any rights.
She has no more rights than any other grandparent would have that you allow her basically.

If her parental rights were terminated by a judge then she is legally no longer your mother which in turn means that she is not your child(ren)'s legal grandmother.