In some cases usability can be bad enough to create a situation where you prefer worse content with better presentation. Sure.
But those are edge cases. No matter how bad a theme of a blog is, on the other hand, it is always secondary to content. There is a journalist's site that I frequent where some of his articles display all paragraphs as a single line. I cut ant paste to TextEdit to read them. He rarely writes an article I don't want to read. Still an awesome site.
