It's only because we don't like the message, that we don't like the messenger.
As others have said, one way or another she is raising awareness of an issue that would otherwise by brushed away by the corporates that will have to make big, expensive changes to their businesses.
Maybe the great barrier reef is fine, maybe it isn't. But if the animals that live there are filled with plastic I'm sure we'd all agree that it's a bad thing.
If the world was the size of your living room I guarantee you'd change the way you consumed and disposed of things, and you would probably do all that you could to generate oxygen rather than filling that room with CO2.
And why would you do that? Because it would affect you directly and quickly, that's why. The problem is that we who exist on this planet right now are unlikely to see the true effects of our wanton destruction of the planet.
A V8, or more manly V12, here and there will not make a difference. A global effort on the things that we all come into contact with every day on the other hand can make a difference. And that's the message that I believe Greta is trying to get across... albeit in an annoying way.