Beau Didapper only has one role in Joseph Andrews: to cause mischief for Fanny and Joseph that will benefit Lady Booby. Sure, Beau also seems to like Fanny—but he's got a weird way of showing it: the guy sees her walking down the road and immediately tries to kiss her. Obvi, she's not interested.
Really, all that we know about Beau is that he's extremely wealthy and "apt to ridicule and laugh at any imperfection in another" (4.9.3). Not to be too critical, but he's one to talk. Between his ridiculous insect-like characteristics and his brash nature, we're thinking that he's not much of a catch.