Some think that good eyesight is all a lookout needs but I see it differently. Usopp is bad / slow in reaching higher vantage points.
In addition, he is not even able to occupy the lookout position while the others are sleeping (see chapter 699). He is unable to keep watch all the time, which is why others also have to take over the lookout position. As a result, they don't have enough sleep. Something that Oda mentioned in SBS volume 74. A lookout that is unable to occupy the lookout position while the others are sleeping makes me doubt that Usopp can be the lookout. He can only occupy the lookout position while the others are awake but not while they are sleeping. The lookout should be able to do both, not just one.
He is not even able to be a lookout outside of sailing.
Since they have the Sunny we have never seen anyone in / on the crow's nest where the person took over the lookout position and reported from there. Neither Robin nor Usopp. Carrot happens to be the exception. We never saw Usopp in the crow's nest at Sunny, off-screen doesn't count. We haven't even seen him at the top of the crow's nest. In chapter 699 he wasn't even in the crow's nest with chopper. After all, that's the lookout position. Has he ever reached the top of the crow's nest? Did he even manage to climb the mast? (Chapter 978) We do not know that. Strange that Oda does not show Usopp in the crow's nest, but a non straw hat active on the top of the crow's nest. Oda had had opportunities to show him in / on the Sunny Crows' nest, but never did. We once saw chopper on the top of the crow's nest (chapter 886 with Carrot). With the one panel you can not see at all whether he has managed to climb the mast. Here too, since they have the Sunny, Oda has not shown Usopp in a single panel where you can clearly see him in / on the crow's nest.
That was it from my side