The problem is, that most famous do not have a superior IQ I guess, so their ideas or thoughts are not better than any other persons!
A lot of the famous would be better of keeping the piehole shut.....Roger Waters is a good example, or any rapper or popstars in general....
The low IQ argument (although you may have been saying it flippantly) is something I have thought about, and it is interesting even if I have not come to any big conclusions.
Firstly, there is some observational evidence that shows people who are locked into their path in life earlier tend to struggle with critical thinking and analysis. For example, sports stars are well known for being - for want of a better word - stupid. Their whole development is centered on becoming elite in one specific field. For many actors this is a similar path.
It's a generalisation that I have not quite come to terms with yet so don't want to go too deep on it. Of course, if your path was centered on something that involved critical thinking (like physics) then your world view may be more expansive.
However, there is no way all people fall under this and some people in the world of celebrity must have a grasp on current affairs and knowledge. The question is where are they? You mention Roger Waters and for me he fits in with Robert De Niro. Someone who is reputed as making thoughtful art does not equate to having something thoughtful to say on other subjects. It's got nothing to do with whether I agree with them or not, they offer such low effort arguments and deal with slogans. They don't even realize Trump saw what they were all doing in pursuing self advertising and seeking approval and took it to the 10th degree and became president off it.
Edit to add that having an IQ higher than the next person does not neccesarily make your opinion on a given subject more valuable.