Actually there are a lot of fans of Usopp and many of those fans would have their fantasies stimulated. Just because some people do not like Usopp, doesn't mean that others don't.
The irony is that Usopp some better moments than Sanji when it comes to the girls post timeskip. After all Usopp has gone where no other Strawhat has gone before and lived to tell the tale then returned there (or very closely on being on opposite side lol) again…. not even Sanji can boast of that
Don't get me wrong, I love Usopp. It's that the Robin-Zoro relationship has been gossiped about aplenty (although, well, it simply doesn't exist).
Lol, I just noticed that Zoro is getting ready to swim in that panel.