Is balding really inherited only from my mother's side?
No, that is a half-truth. A major hair-loss variant does sit on the X chromosome that men inherit from their mothers, but more than 200 markers from both sides of the family contribute.
The old rule captures something real but oversimplifies badly. One important androgen-receptor variant linked to male-pattern baldness does sit on the X chromosome, which a man inherits from his mother, so the maternal line genuinely matters. That is where the folk wisdom comes from.
But male-pattern hair loss is polygenic. Large studies have identified well over 200 genetic markers associated with it, spread across many chromosomes, and they can come from either parent. Looking only at your mother's father tells you part of the story at best. If you want a real sense of your odds, look at hair-loss patterns across both sides of the family, not just one.