For a long time films that would be considered niche and arthouse were middlebrow, because film itself was at best a middlebrow medium.
To people still concerned with the various brows, Marvel films are below low. They are sign of a debased and infantile film culture that caters to childish tastes and merchandising, not art.
Master and Commander is pretentious pulp. Real, quality media is obscure, and largely unpalatable to our debased modern sensibilities.