Given the fact that Usopp is still there helping the dwarves when he could have come up with an excuse to stay back, I don't think he's that much of a coward. Plus, it's normal to see Usopp be scared, but he almost always delivers when the time comes. His convictions can be strong when his crew mates are threatened in some way.Maybe the difference isn't how much of a coward he is, but how Oda shows it to us.
Usopp has always trembled in fear and tried to skip fights with ridiculous lies, but up until the timeskip Oda kept portraying this in a consequence-free manner. He may have acted scared and cowardy, but that was usually done with great comic effect and at the expense of other Straw Hats whom we knew would save the day, unless he himself manned up and did the job.
Whereas here in Dressrosa, for the first time we're seeing Usopp go through wih the coward act. He's not only claiming to be afraid, but also manipulates a stronger party who isn't a Straw Hat into taking the risks in his stead. For the first time, we're looking at the real prospect of Usopp not doing his share of the work. Yet unless he actually turns his heels and abandons his responsibility, he won't have been a bigger coward than usual. His acts have simply caught up with his words, which we might want to consider as better characterization.
Though, I am annoyed by how cowardly Usopp is acting. He's never been that cowardly, but I am confident he will help the dwarves when they're in serious danger.
He's basically like a yo-yo or a boomerang. There will be moments when he runs away due to fear, but he always comes back to show that he does have the attribute of courage- it just needs a little push.