You're so close to getting it. You literally just used the phrase "some people" and still can't grasp why the joke is transphobic for making a broad generalization about the desirability of trans women.
Because you're exactly right: some people don't. But other people do. Which is why the joke is transphobic for implying that the reader will immediately agree it's "bad news".
And don't give me some shit about the author and their characters because you and I both know that these are one-offs and the joke is "uh oh you got duped that girl is actually a guy hahaha"