Tesla and SpaceX have done great work. Its hard to tell exactly how influential Musk is to these companies. Perhaps a lot, perhaps only financially, perhaps just a figurehead. When his tweets are less than brilliant, your prior assumption starts to move away from brilliance and towards the idea that he's just a blowhard at the top of two great companies. Couple that with the fact that Musk makes a lot of broken promises on Twitter you can see why people think Musk's tweets detract from his reputation.