no, the waveclearing potential of rumble isn't really higher than luxs, as it doesn't matter how much more damage his ult can deal if Luxs already is enough to vaporize the entire wave.
It's not like overkilling a minion wave will transfer the remaining damage to the next wave spawning or something...
More on the subject: Rumble got his ult cd nerfed back when he was strong 'because he could use it for waveclear and still have it up for nearly eveyr major fight'
And lux currently is pretty opressive, and she uses her ult rather frequently for waveclear while still haveing it up every fight.
So people complain about lux because her kit is rather bulshit to go against while she is simply to strong atm, so they want nerfs.
And since Lux has the exact same pattern that got rumbles ult cd nerfed, many jump to the conclusion that nerfing her ult cd thus would be a good idea.
I think the ult cd would be a bad spot to nerf her, haveing a rather spamable longrange snipe is imo an important part of her identity. I think the main problem is how easy her E is to land, how strong her Q is since the buff, and mostly how much tankiness she gets from her shield.
As a champ with that much cc and range, she should atleast be squishy if you get to her, but she simply isn't.