It's not true, though. SF is pretty much average in this regard.
Americans in general just don't seem to care about public spaces, pretty much anywhere in the country. I've travelled to many US cities and the only reason other cities some how feel "cleaner" is because there's literally nobody walking in them.
The cities made for walking (like NYC and maybe Boston) are on par with SF. Maybe less dog poo everywhere though.
This reminds me of my earlier experiences walking around LA. My natural instinct looking at the state of disarray in some of the streets was that I was on the wrong side of town, because where I come from (Granada, Spain) that's what you notice when you venture into the slummy areas. In LA, however, you need to readjust your gauge, as this was a fairly safe area, just not well taken care of.
SF has an extreme homelessness problem, smells like urine in upscale pedestrian magnet areas (not just back-alleys) and the residential buildings are poorly maintained to the point it being visually distracting because the land underneath them is so much more valuable than the edifices themselves.
NYC is better, but still fairly grungy. Boston is substantially better along these metrics. Montreal and Quebec city, two of the oldest and, by dint of age, pedestrian-friendly cities on the continent, are leagues ahead of SF as well in this respect.
SF is great. Love the vibe. But lets not kid ourselves into thinking it is clean.