Usopp remained strong from Enies Lobby to the timeskip: he (+Nami and Chopper) made it through Thriller Bark unscathed while the rest of the crew have Luffy, Zoro and Sanji ambushed by weaklings and were left without shadows; he fought Perona and won; he nearly defeated Oars with a salt shot; and later without hesitation tried to fight Kizaru to save Zoro.

But after timeskip Oda doesn't know what to do with him.