Rainbow Dash smells bad because she doesn't wash much, but Twilight and Fluttershy still like her, and Twilight isn't even afraid to go near her anymore. Her shorts and armpits are the smelliest parts of her other than her feet. And she takes pride in her smell.

Another Smelly Rainbow Dash pic by request. If you dislike the concept or edited screencaps, please ignore. They're not meant for you, just people who like this kind of thing.

As usual, I don't think she's really like that in canon. She probably washes every other day at the very least. Though she probably does find it fun to fart, which she's not doing in this pic.