He's all grown-up now and she feels proud (and a little responsible, although not so much) any time he meets with diplomats and impresses them with his amazing wit and keen insight. Or any time one of his inventions solves a little village's problem. Or whenever she visits a town that sells souvenirs of them and they talk about that really smart Water Tribe boy who is a man, a very smart man now.
But she also feels proud when he makes a groan-worthy pun and the diplomats laugh out of embarrassment. Or when he makes toys for the kids and ends up playing with them alone. She even feels proud when he shows his drawing skills are still somewhat lacking.
She feels proud because changing and maturing is hard and painful and admirable, but to mature and still remain the same idiot is something few people can boast about.