That makes no sense lol. It's not like Usopp is going to fight Trebol and/or Sugar for his own goals or anything like that. He's doing it for the dwarves( it looks like he doesn't even care about destroying the factory or Law's plan that at this point)
It seems like you are completely misunderstanding what Usopp said in the chapter, please read it again. He's not fighting for the dwarves. He's fighting for himself and his reputation. He just said that he couldn't sleep at night if he let this happen to the dwarves. He's not a hero, he's a pirate.
Also, I think you guys are confusing cowardice with being a liar. Usopp is both and he wants to get of the former. Him being a liar and calling himself "Captain Usopp or Usoland" has helped him TREMENDOUSLY so far and it's the only reason he's even alive at this point. He's weak but he's intelligent enough to trick and confuse his opponents with his lies. It's his most powerful weapon and that's what makes him interesting. Why would anyone want him to get rid of that is beyond me.
I agree with you here that Usopp definitely does not need to lose his skill at lying. However, I will say that Usopp is beginning to realize just where he should draw the line. As you put it, it is his most powerful weapon, and who has it hurt this time?
I know you want to answer, "the dwarves!" But that's not the point. He's hurt himself with his own lies, this time. He hurt his own pride. That's why Usopp is developing character right now.