Usopp not wanting to help the dawrves is a "normal" reaction, a "normal" person, let alone a "normal" pirate wouldn't put themselves out in harms way to help solve a situation that has nothing to do with them. Not everyone can be as reckless as Luffy or as selfless as Otohime/Vivi/King Riku.
And that is why "normal" Usopp's reaction is fine. I'm sure the heroic and courageous Usopp within himself is struggling against his current cowardice and hesitation that has had two years to build up. There is currently a wall representing Usopp's lack of belief in his ability that is blocking Usopp from his "real" self. Usopp not being in situations deeming him to step outside his comfort zone or at least far away from it over the past two years has led him to regress back into his former self before he met the Strawhats (one where he hides behind his lies). Usopp may have physically gotten stronger and upgraded his weaponry and ammunition, but the mental aspect that has also been Usopp's problem in the first half of the Grand Line isn't that easy to train and change.
Usopp having recently rejoined with the rest of the Strawhats (what has it been, 3 or 4 days since they all came together?) is currently struggling to find his self that had grown accustomed to danger by the time the Enies Lobby Arc had crossed the Strawhats path. What Usopp currently needs is a reason to break down the wall holding him back, reminding him of what he is truly capable off and consequently allowing himself to unleash. Or for his courage to built up to the point that explodes through the wall blocking him.
Usopp's ability/strength is inversely proportionate to his cowardice, that is to that say that the stronger his cowardice, the stronger his fighting capability when it rises above that cowardice.
Whether it be through the Sogeking mask, or his own decision, or seeing Robin injured, Usopp will rise to his potential in this situation, especially after Oda has built him and his role up so grandly. Usopp is currently the Strawhat in the most key position for seeing Doflamingo's plan truly failing. By taking out Sugar, Usopp is effectively pulling out the card within the Donquixote House of Cards keeping it stacked. Once Sugar is gone, just like when a house of cards falls and crumbles, so too will Doflamingo's plans he had meticulously built up over the past ten years. With the truth being revealed, chaos will likely arise across Dressrosa consequently affecting Doflamingo's state of mind as well as the safety of the SMILE factory. Both Marines and pirates/criminals will be after Doflamingo.
Usopp not being seen yet by the Donquixote Pirates must have a reason to it, as well as the build up Usopp has gotten since meeting the dwarves. It is likely Usopp will find his courage current dormant from the lack of use over the past two years, but once Sogeking/Usopp has been unleashed, Doflamingo's end will truly be in sight.