if you're looking at a catholic baptism, i believe you'll need at least one godparent that is from the catholic church (not necessarily active in the church, but is a confirmed catholic). this is my understanding of the catholic process. i would go and speak to the priest at your local church and ask them what their "rules" are as such.

even though DH and i are not active in the church, i am a confirmed catholic. i do struggle with some of the stances the church have made and as a result, i don't attend church, but i do have my beliefs, and our children will be baptised with catholic godparents. DH is not catholic, but given our choice of godparents (bro and sil) he agrees that a catholic ceremony is appropriate. our children won't be raised attending church, but will be bought up with christian beliefs. if, when they are old enough, they wish to attend church, i have no drama with this (it's next door!) - but it will be their choice

if this is the way you're looking at it, definitely speak to the priest about what the expectation re god parents is - and if you want your mum as the only godparent, so be it. give his parents another "special" role in Mia's life!