So, I think the totality of the user reviews here suffer from two things, one, sampling bias, and two, biased judgment about the subject matter. Sampling bias I'm referring to the fact that the few people (in the world) who watch this and able to write a review are from the bodybuilding community, who knows a thing or two about the actual history, so they are not necessarily the best "movie reviewers" and probably think too much about how it matches real history, biased judgment goes to the reviewers who has some negative opinion about bodybuilding in general. In any case, if you forget all that, this movie is actually a well paced, fun and moving film to watch in its own right.
For sure, one may ask "What's the higher cultural and social significance here?" Answer is maybe not much, but, the writers know that, and they have done a good job dramatizing the series of mini/focal conflicts and struggles, so each one of them is fun to watch as they occur. Overall it's a linear and straight to the point "documentary" of probably made-up series of failures and successes, and the character background (mother wanted a girl, mother against weight lifting, being a jew, weak) are not beefy enough, it managed to capture attention and when Joe struggled to show emotion in front of Betty, it worked.
Overall, the acting and constructing of a character who's almost possessed and highly focused on one strange thing stands on its own and makes the character fun to watch, the acting is pretty good too.
One last point I want to make is "so what it is about boasting the success of the Weider empire?" aren't all biopic success stories about boasting the protagonist? Watch it as a motivation story.