I've seen a few comments here and there since the time skip about how Usopp is still a coward yet to me I have noticed a definite shift. Pre TS Usopp had a more childish fear. He was skittish about most dangers whether genuine or perceived and his expansive imagination didn't help either. He also lacked confidence in himself and was really quite weak anyway.
Since the TS his fears are more grounded in reality. He has grown but so have the dangers and before when it was more of a jaunt now it's very real and very life threatening. So for me Usopp is not afraid for the sake of being afraid but because he knows he should be. Of course you can't change your personality or a lifetime of reacting a certain way so I don't blame him for having his fear overwhelm him but it's much more understandable now and he's not so flighty in his reaction.
Erm I'm not sure how better to describe it other then Usopp has gone from imagined childish fear to recognising genuine threat and fearing that instead.
err like as a child you feared the bogeyman but as an adult you feared the burglar