We've all seen/heard how Trump "leaves things to the experts" on this issue as you suggest, he says they are all (97% of the worlds experts) are basically full of shyte and he knows better. So what I hear you saying is Trump is not more mature than a ~16 year old. I would agree.
If Trump doesn't believe in AGW then he's willfully ignorant of the science. But again, neither Trump nor Greta have enough expertise to make substantive claims about climate change. That Trump doesn't listen to the broad climate science community he's a fool.
I guess Greta is more a symbol of a movement more than anything.