Remember what Sugar's power is? If so, then do you remember it's side-effects? If you do, then bear with me.
Robin got turned into a toy.
Let that sink in for a moment. 3 seconds will do.
All done? Now, which 2 arcs has Usopp grown the most in? That's right. It's Arlong Park and Water Seven/Enies Lobby.
Which arc was Robin's arc? Oh yes.
It doesn't stop there. Remember this page?
http://www.mangapanda.com/one-piece/740/19
http://readms.com/r/one_piece/740/2279/19
[Powermanga link here but it's not available yet]
Why is Franky calling Usopp as Usoland? Doesn't that strike you as odd? Now, when did Franky first meet Sogeking/Usopp as an ally?
Here's my theory: Because Robin turned into a toy, each of the SH members forgot about her and everything related to her. It doesn't change who they are and what they're trying to accomplish, but I'm guessing that their memories related to Robin are either completely fuzzy or forgotten.
Like say the first 5 Strawhats first met Robin when she 'killed' Igaram. Maybe their memories are altered to the point where what they remember is that Igaram was victimized by a bomb by himself, and the culprit is still at large. Fast forward to Enies Lobby, and maybe all they remember is that they ran wild on a secure government installation and kicked Lucci's ass just for the fun of it.
Now, everyone is pissed off at Usopp, right? He seems like a different character, right? Like he regressed back to pre-Enies Lobby development, right?
I think it's because his memories relating to that arc are gone, and with it, his character development. He doesn't remember why he has to help the Tonttata and why he promised them that he would catch them when things got tough. All he knows is that Franky isn't there yet to save him, and all Franky remembers is Usoland, and not Robin.
Sugar just undid Usopp's character development by turning Robin into a toy.
This is all a theory though, trying to rationalize why Usopp did what he did.
EDIT: Look, Usopp's always been a coward. That will never change. However, his actions after the time skip have all been due to self-preservation. Going against Doflamingo is a bad idea, let's just not attack the factory, oh no I have to guard the ship else Doflamingo will attack, etc. Last chapter too, he told Leo that they should all retreat and pull back. "You need to get out of here! There's nothing more you guys can do!" The bolded should tell you that Usopp was brave enough to be concerned about his mates even after the explosion.
This time however, it seems that he's fleeing because of fear. He left everyone behind. Does that ring a bell? Arlong Park, maybe? How about Water Seven, when he left because he didn't think he was strong enough to tag along with the crew? One incident does not make a person brave. Also, every time Usopp's been brave before Enies Lobby, there was always someone with him (except the Chuu fight).
Does that sound like the same person who donned a mask just to rescue someone from a famed government installation? Does that sound like the same person who took on a ghost girl who no one could beat but him? Does that sound like the person who was dripping swag all over the place because he'd faced a digging enemy before?
It all boils down to whether or not he forgot about his resolve when he went and rescued Robin, and I really think he did along with her identity.
EDIT 2: Here's how Sugar's power works. After she touches the person and that person turns into a toy, everyone will forget him/her and everything associated with that person, which would include events.
http://www.mangapanda.com/one-piece/728/18
http://www.mangastream.to/one-piece-...9-page-17.html
[Batoto is not working for me]
http://onepiece.wikia.com/wiki/Sai
If Chinjao forgot about Sai but didn't forget the events associated with him, he wouldn't have said that he wanted someone to inherit the Happou Navy, because as you can see, Sai already has command. Chinjao would have memory of someone inheriting the position, it's not just that Sai did.
But he doesn't remember.
So that means everything related to the person will be forgotten. Cue the Strawhats forgetting about the whole Enies Lobby incident. They'll just think that, "Hey, we just went there to kick some ass!"