The problematic part is that Mr Beast offers little if any critical reflection about the systems that create wealth inequality, pollution, etc. and even less regarding his own role as a perceived philanthropist within that context.
It's not that it's inherently morally wrong to clean up the ocean, open a free bank or handing someone 500k for staying 100 days in a 100ft circle. It's the manifest obliviousness of not challenging the system and the premises that allow him to make his videos that's - industries generating single-use plastics, people not able to take out a mortgage,... - which is the issue.
His videos aren't in any way a social commentary. They are nothing more then entertainment, and they don't pretend to be anything else. That's what makes all of this morally questionable.