ANY group can be obnoxious and terrible annoying. I've sometimes been embarassed by groups of countrymen totally oblivious to the other visitors.
I've been annoyed at groups of French schoolkids in museums, German groups at beaches or pools and yes American groups too. Honest, do they really need to compare everything to stuff back home and then complain that at home it's bigger/cheaper/cleaner/better??? And do so LOUDLY??
Many people forget that when they are in a groups they don't need to always stand together blocking roads, stay together in line or leave some space for others.
That's really not limited to Brasilians Or Americans or whatever nationality....
I have met many, many nice, kind, intelligent Americans/Germans/French too.
But groups are a reason we took hoppers
That way we can cut and run when one park is overrun....