Why doesn't Usopp create great weapons for himself?
He does.

Not only does Usopp have hammers (which are quite modest), but he uses a variety of gadgets to help him though most are ineffective and funny. He uses Caltrops, fingernails on chalkboard, grapling hooks, suction cup shoes, and fireworks. He also uses several dials from Skypeia but these are few and far between.

The more impressive weapons are his slingshots.  He has had three.  The first seems to just be a normal slingshot but he made the latter two and they are amazing. Kabuto uses dials to accelerate the shot fast enough to snipe targets a long distance away. Kuro Kabuto is actually a living more dynamic version of Kabuto.

Finally, Usopp's lies are his main weapon but that is not really relevant to this question.

The impressive thing about Nami's Clima Tact is the part Usopp can't replicate. It originally contained a series of gimmicks and tricks that seem to make sense as the kind of silly thing Usopp would make for himself. Nami was able to use that in very unexpected ways with her weather knowledge and amazing intelligence (third highest in the history of the East Blue which is about 1/5th of the entire world).  Later versions of the Clima Tact have more of her input so are less silly.  They depend on her weather and scientific knowledge which Usopp simply doesn't have.