Yes, this was at one time a rumor. Though, I don't believe it ever had any real proof behind it. The name "Nimbus" has been around in Mortal Kombat ever since MK1. Originally, there was a supposed character called "Nimbus Terrafaux" hidden in the game. He was an African-American kickboxer, and the rumor likely began because mk1 contained no African Americans. Personally, I believe the MK team used many of these fabricated ideas and made Jax for mk2.

The name 'Nimbus' mysteriously came back for some reason, I did hear of this rumor about someone connected to Noob Saibot. Not his relative exactly, just another member of a group he belongs to, "The Brotherhood of the Shadow." I believe, something along those lines. Perhaps the person who started this rumor found the name 'Nimbus' to be cool in a mysterious sort of way, that would be my guess.