Nick does not attract boring people as much as he attracts people who want to tell him their story, which ends up being a tedious burden.
On the first pages of the novel, Nick offers a description of himself as a person who is usually neutral and who reserves judgment. His neutrality is taken by some as a political stance - "I was unjustly accused of being a politician" - and taken by others as an invitation to confess without fear of being condemned.
This quality in Nick, that of reserving judgement, is what draws Gatsby to confess his story to Nick. At least, this is what Nick attempts to imply in his introduction.
Due to Nick's willingness to apply an "infinite hope" by reserving judgement, he is subjected to many stories, not just Gatbsy's, and he has become quite tired of hearing them.
Nick claims that he attracts boring people because he is a quiet man who listens to them and does not judge them outwardly. Once we keep in mind that Nick is an unreliable narrator we can better determine if he truly attracts boring people; or if he is simply bored of them.