This is supposed to be funny. He says that his detective skills are similar to the detective skills a dancer would have. At the same time, dancers are not expected to have detective skills at all, so in a group of dancers he would be considered a great detective, since he does have some detective skills after all.
"For a dancer, ..." would normally be followed by some comment about his dancing. By following it about a comment about his detective work, he implies that there is nothing good to be said about his dancing.