An ugly site can be more trustworthy - "this is made by a person, not a professional company that doesn't do anything without the purpose of taking my money"...
So, ads on ugly pages can similarly be more trustworthy to some users, that think of them aas recommandations from a "little guy".
Also, sites that look "too good" sometimes compromise in respect to usability (pageload, simplicity, navigation, not enough text etc.) - making them "less sticky" than an ugly site.
I disagree with ugly = trustworthy. In fact, so does the research. Dr. Fogg – the author of Persuasive Technology: Using Computers to Change What We Think and Do – reports that "the design look of Web sites was clearly the most prominent issue when people evaluated Web site credibility in this study. Almost 50% of comments about Web credibility contained something about the look of the site, either generally (e.g., ‘looks professional’) or specifically (the layout, the colours, etc.).