She didn't say it, but I kinda feel like "gap moe" was also part of Kaoruko's answer. Rintaro being a truly kind and genuine guy despite looking intimidating holds its own appeal to some girls, I'm sure. He looked like a bad boy on the outside but acts and speaks like a caring good boy with a true heart of gold. If she wanted to feel like she hit the jackpot finding him I'd say let her.
Kyoko holding back her tears, she must've thought "My little boy isn't just grown up, he is whole again." She has watched him drift quietly through adolescence, and now someone has managed to bring sunshine back into his life. And this same girl is now here because she and her husband raised Rintaro properly.
A bit disappointed his dad didn't at least show his face, preferring to send his regards via cake. Maybe Mr. Tsumugi really is just that awkward. I guess this serves as his dad's approval of their relationship. No direct words, but maybe it got across to our little miss cake lover through the heartwarming goodness of this special cake. "If you wish to be with Rintaro, I welcome you to our family"