There's pretty complicated dynamic regarding ad quality because each participant (users, advertisers, publishers etc) have different goals and ad network needs

There's pretty complicated dynamic regarding ad quality because each participant (users, advertisers, publishers etc) have different goals and ad network needs to balance it. For an advertiser side, one possible, measurable metric is advertiser value per spending money. Value can be defined by advertisers pretty arbitrarily but you can think it as something like how much money you earn from ad conversions. Of course, this is very hard to measure accurately, massive ad network like Google has some advantages though. But note that this is just one of possible metrics, there are hundreds of metrics ad networks want to improve and some of them are conflicting each other by definition. And some of them don't even have an objective way to measure so still depending on user study.