Wow I really like that video. Truth is, she looks good, she carries her weight well. I think she should be happy too
As for if fat people can be truly happy - it depends on the individual. You can be skinny and miserable, so why not bigger and happy? She feels good, looks good, and doesn't feel she has to conform. Good on her tbh.
I don't wanna be any smaller than a size 10, and although I know that's fairly small, I'd never want to be a size zero, even if everyone else decided they did :S It's about getting to a level that you're personally happy with, and if you're happy when you're what society considers to be bigger, than fine.