I recently noticed a Japanese comedian named
お侍ちゃん. It indeed sounds funny. Why is it?
I am not a native, but I would make two guesses as to why it sounds funny:
1) It could be because ちゃん expresses that the speaker finds that person endearing. Since お侍 is a position that holds superiority, the use of ちゃん now becomes condescending and rude.
2) The other reason I can think of, is that in japanese the combination of お and ちゃん that I have seen are applied to the elderly while still holding the endearing quality such as おばあちゃん and おじいちゃん. Substituting 侍 in place could show that their still maintains a characteristic of being amicable between the person and speaker regardless of the 侍 title.