Indeed. It can be a reinforcing cycle either way. On the one hand, you devalue your ads by taking any offer that comes along, ignoring the effectiveness of ads, and cramming your content with as much ad space as possible. In this mode readers have no value for ads and ad space becomes less and less valuable. On the other hand, if you put a lot of effort into procuring good and relevant ads, if you maintain a high standard for the ads you accept, and if you keep ad space under control then ads can potentially become more and more valuable.

If you come into the problem with the idea that the only way to control ad revenue is by the number of ads on the screen you've already lost, you're playing in the amateur leagues.