I was in the U-District 2013-2018. It varied on and off with how dirty and how overrun with the homeless/Ave Rats it got, but it wasn't noticeably worse than any other busy neighborhood in Seattle. Generally though, more empty storefronts = more dirty because of more spots for the homeless to spot up and loiter/camp.
Amazon killing the small retail store market already took out so many businesses on the Ave in the last decade that it created a situation of more storefront supply than demand could fill. There's a limit to how many restaurants can actually be successful and turn a profit on the Ave, so there's insanely high turnover in that genre. I'd say over 50% of the places that were open my freshman year don't exist anymore. Some storefronts changed to different restaurants 3-4 times over the course of my time as a student. It's a brutally competitive scene.
I went back to the Ave a few months ago, and it was pretty evident that COVID plus a whole year of no students on campus completely fucked over the businesses on there. So naturally, with so many dead and empty storefronts, it creates an environment where it's dirtier and more overrun by the riff raff now than ever before.