I haven't seen Top Gun, but I can directly say it's a "cool American pilots" movie. I have seen Avengers, and Captain America is the "Big Brother USA" person. Why is my first opinion invalid all of a sudden?

both opinions are correct but whether a secondhand opinion is valid depends on how surprising it is. it's universally accepted that Top Gun is a "cool American pilots" movie. If you claim that it's a subversive and erotic Marxist noir masterpiece that would be ridiculous and you'd need to back it up by having seen it and being able to give specific examples and stuff. I haven't seen it but I think there's a fairly large audience that just likes Barbie unreservedly so to criticize it effectively you need to have seen it.