How to disable “Top Hits” in Mail
Unfortunately, Top Hits cannot be disabled. However, it should only show up when your search is rather vague (i.e., searching for the word "the" would surely trigger Top Hits). The more specific your search is, the more likely your results will NOT include Top Hits. Be careful, though - sometimes trying to be too specific may yield no results at all! When I search my email, I try to use as few search terms as possible and always include words that are distinct to the content I'm searching for.

I realize this probably isn't the answer you were looking for, but hopefully I've helped you minimize the likelihood of Top Hits showing up when you search your mail.