Would you think a person still be justified in thinking that cartoons are all dumb and childish, even though the marketing presents it as that?
Yes, they would be entirely justified - especially but not solely if the thing being advertised hasn't come out yet and they have nothing to compare the advertising with, but even afterwards, assuming they take in other information about the thing (reviews, word of mouth, snippets they happen to catch while browsing (though the last one is becoming rarer with the move toward streaming and the need for viewers to affirmatively choose what they'll watch)). Marketing is supposed to influence you. You are justified in being influenced.
Others might think your opinions are wrong, and your opinions might very well be wrong, but that's true of anything. You're not more justified in having or expressing opinions just because you have more information, you're just more likely to be correct when you make affirmative characterizations about it.
Take an example. I haven't seen this movie, I don't plan to see this movie, and part of the reason I don't plan to see this movie is because I find the project it's part of too nakedly cynical to ignore, because other people, both reviewers and acquaintances, have told me that other, similar movies aren't terribly well-crafted, and because nothing I've heard about it specifically (almost exclusively from this thread, both from before and after it came out) has convinced me that it would entertain me. That is the alpha and omega of what I think about this movie (setting silly riffs from earlier in the thread aside). I don't know if it's well-made, clever, heartfelt, or politically correct (I'm using this term not as a slur, but actually to mean having good politics), likely won't unless someone tells me, and am not in a position to speak on that matter. So there are limits to a purely marketing-based take. They are not very valuable as a source of information on anything other than one's inner life and how movie marketers might appeal to one. That doesn't make them illegitimate or improper to express publicly.