Most of the top critics do approach these movies with the proportionate amount of respect that a Summer Blockbuster deserves. Roger Ebert, especially.
Not to mention, even if they give the movie a fair shake, their reviews operate from assumptions about comic books NOW based on the comic books they read as a youth, or outdated assumptions of what the genre can and should accomplish or has accomplished.
For example, even if Roger Ebert is entirely correct about his criticisms of movies like Spider-Man
or Fantastic Four
, he consistently has a problem with supers not being super. The Fantastic Four don't do anything really fantastic (like move continents or reverse planets) and Spider-Man's wimpy rejection of his own feelings for Mary Jane (which he compares to impotence).
He still comes from the school of "superheroes must be larger than life, mythic and grandiose and maybe even perfect" that isn't necessarily wrong, but is simply not a objectively useful way of measuring the narrative merit of a superhero film. Which is strange, because he's perfectly willing to withhold that yard stick for other 'super films' if they look like they have enough gravitas.