It's actually not ok to have an opinion of a movie you haven't seen.
But that's pretty how things work. That is how people decide which movies that they will go see. They watch trailers and the studios attempt to put their best foot forward to entice movie goers to buy a ticket. And people generally decide if a movie looks good to them or not, before seeing the film. There are obvious other factors, but it ultimately comes down to if you feel like seeing a great/good/mediocre/shitty movie or not. And those assessments are perfectly fine to make.
A prediction is fine, it's people who claim they know the movie is terrible to the point that they root for it to be bad, cuz they'd rather avoid having to admit they were wrong than have an awesome Venom movie to enjoy.
People did the same exact thing with Homecoming ---