Over the next few hours, this change should propagate to all users:
We've reduced ad coverage on the forums by about 25% on mobile devices. I've received complaints and also been bothered by how prevalent ads are on phones, specifically on the forums. While there will still occasionally be ad-post-ad situations, it will be rare now and after testing this for several days, I've seen more instances of four posts in a row with no ads between than I've seen ad-post-ad situations.
Hopefully this makes browsing and participating on the forums a more pleasant experience on mobile devices!