It doesn't have to make it worse, just not make it better. Almost no one ever says "I love using such-and-such a website, the adverts on the side are just so relevant".
When I'm looking for something, relevant ads are great. Ads that were relevant to something I was looking for a month ago? Not so much.
When I'm looking for something, I'll usually have already done a thorough search around on google, ebay, amazon etc... and it's unlikely that targeted ads will show me something I've not yet seen.
If I'm looking for something I want to read trusted reviews of it, not ads.