I’m getting the impression that few, if any, Thunderians would side with Grune.
I’m not entirely sure Mon*Star would make an appearance in the “current” time as Lion-o. He was shown in the flashback that occurred, what, generations ago? So he would have to be a few hundred years old by the time Lion-o and the gang are walking around.
It’s possible that Mon*Star’s teachings were passed down through the generations who revere him as a prophet of some kind.