Maybe your subjective opinion is different than other people's subjective opinions?

In fairness, mediocre marvel movie is absolute bottom of barrel.

For comparison, The Room got a 9. Maybe metacritic scores follow a bathtub curve with "worst possible" sitting somewhere in the middle?

Distribution of grades is rarely even, and I'd guess that if you looked at the median grade on Rotten Tomatoes is would be much closer to 70 than 50. Things less than 50 are usually "very horrible".