At best, one can prove that the ad was served to a computer, not actually viewed by a user. Unless there's an incentive for the user to view the ad, as there are rewards for mining blocks, this model probably wont work. Granted, there are ways to incentivizes the user, such as paying users to view ads, but this kind of defeats the purpose of getting the user to pay your and doesn't really require a blockchain.

How would this work? Ad-viewing is a client-side function.

The device where the impression is served would do a little work, proving that the ad was indeed viewed and thus preventing fraud.

Is that even possible to prove?

Yes, but only if the viewer proceeds to buy the advertised item.