I normally use IMDB. Quick tip: Never check the average rating of a movie. Instead immediately go to User Reviews and check the spread of ratings given there.

You'll find that many 'blockbuster' movies with high average rating have reviews with lotsa insanely high 10's and lotsa 1's on the other hand. I've found the 1's are more trustworthy overall (plus, if you don't mind spoilers, there are hilarious plot analyses to be found).

I think you're right that averages are not useful when scores are bimodal since that basically means there are two distinct populations being observed, and what is more important is what population you belong to.

I think the existence of this bimodal distribution is at least consistent with the "haters gaming the system" theory, though not conclusive.

The IMDB ratings for the last jedi do not seem to exhibit such a strong bimodal distribution: https://www.imdb.com/title/tt2527336/ratings?ref_=tt_ov_rt

Which I think leads more credence to the theory that haters are overrepresented in the rotten tomatoes numbers.

That's really good information. I've stopped trusting Rotten Tomatoes not a long time ago. To be honest, I have stopped taking any "average" ratings for anything. At one point, I trusted those ratings for everything related to entertainment: movies, games, touristic places. But the truth is that the ones affected by reviews, that have power to influence it, will skew the perception, interpretation, and numbers of those reviews.