After seeing this question, I began to wonder if the Nimbus race were featured in any other Mario games. I can see a common cloud theme throughout Mario games ranging from background clouds with eyes to Lakitu's Cloud. Does anyone know of any evidence that these are related?
Nimbus people haven't appeared in any other game even as an easter egg to my knowledge. They certainly haven't become as abundant as say Lakitus or other series mainstays.
A single lakitu appears to be in the thrall of King Nimbus, the "Royal Bus Driver" (that's a bus?). However there are Lakitu all over SMRPG and most do not appear to have any affiliation to the Nimbus people so I wouldn't jump to make any association beyond "Square/Nintendo thought it made sense for a Lakitu transport in a cloud land".
Note that, possibly due to Intellectual Property issues with Squaresoft (now Square Enix), almost no characters/species introduced in Super Mario RPG have crossed over into future Mario offerings, despite Paper Mario being a perfect opportunity as a somewhat successor and frequent fan requests for Geno in Smash Bros.