The ‘brow’ standards have dropped significantly, in a process Fussell has described as the general proletarianization of culture.
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.
Years ago I was surprised to read a critic that described Branagh's Hamlet as middlebrow. I mean, Henry V, sure - that only even qualifies as middlebrow because it's Shakespeare. I would assume it was lowbrow at the time it was written. I love the prologue, though.