The only pony I could possibly have a child with is Pinkie Pie because she would break the rules of logic, which is required to have a baby with me.
Transman problems....
But if I were cisgender and could impregnate Rarity, I don't know if she would make a great mother. She has a lot of fabrics, there is a room in her house that looks like a huge mess, she has a lot of needles, scissors, and a lot of other things not safe for foals. Knowing how children are, even if Rarity put away her stuff, the foal would figure out how to get it.
Rarity's environment wouldn't be safe for a child, she would be great at raising a foal, but we would have to put so many safety precautions so we don't have to take our foal to a hospital and explain that they ate needles that were in Rarity's top shelf and locked away. But besides the safety issues, Rarity would make a great parent.