Everyone at their positions excels. More than likely they will be considered the BEST in the world at their ship roles when it's all said and done. So Zoro being considered the BEST lookout doesn't seem plausible. Hence why lookout and helmsman are used as possible next positions. But I'll partly drop the lookout position…I say partly because NIGHTWATCH is a lookout but specifically at night. I believe they do need this role on the ship. This nightwatch could also be an ASTRONOMER(see the connection?) as there is already an astronomer's telescope in the crows nest. Zoro isn't using the telescope so who is? Which could provide Zoro some company(training or drinking buddy?) from time to time as I stated before.
A nightwatch is not really necessary as the crew is known for making shifts.
Here, Franky is doing it just fine. Zoro also barely sleeps as his bedtime is just a measly 3 hours (1:00 AM to 4:00 AM).