The smell of a girl's asshole is closely tied to her lifestyle choices. Just looking at how in-shape she is, you know she's not the kind of trashy chick that eats like garbage and has a digestive system shot to shit. She likely always produces healthy, large, solid, bowel-emptying poops that, while inarguably smelly, aren't the kind to make you gag and want to vomit.
She definitely isn't a fan of daily showers or frequent laundry. Unless she uses a bidet, she will often have to rely on solely TP to keep the space between her cheeks clean, which isn't a problem for one or two days. But for any longer, adding on her active lifestyle and large bowel movements due to a high-protein diet, and a slick, paste-like layer of sweat, pubic hair, dead skin cells, and trace amounts of the bits of fecal matter TP can't cover will begin to develop on the surface of her anus.
In the end I think her ass will smell like what it smells like in the bathroom after you'd just flushed your poop and forgot to open the window - moist, musky, sweaty, and with the unmistakable cloying but muted odor of feces. But I think the smell of sweat alone would nearly overpower even that.