When the trolls are singing the Fixer-Upper song one of the lines is:
Are you holding back your fondness
Due to his unmanly blondness?
Why would they consider blond hair "unmanly," or consider this a big enough concern that Anna wouldn't want to date/marry him? At first I thought I might have heard the words wrong, but these words were confirmed by three different lyrics sites.
Here is the scene:
The lines in question happen about 1 minute into that video. The trolls are also clearly poking fun at Kristoff so maybe this was just some kind of absurd claim to elicit a response from Anna.