No that's not my opinion. A "bad" film has a IMDb score of, typically, below 5.0.
A lot of what went on in Prometheus has been speculated on for decades. And a lot of that speculation turned out to be true. I think that's why some people are fundamentally disappointed with Prometheus. Because it was too predictable i.e. it went down the route that they always suspected it would. Then there's people that thought they were going to see a "sci-fi horror slasher" movie like Alien and were left disappointed. In reality it is just a pure "sci-fi" movie.
Historically, films that try to wrap fiction around the origins of the human species tend to be hugely divisive just like Prometheus has been. Think back to when Stargate came out. It got panned, and even today it still only has a 6.8 on IMDb. And yet in any sci-fi buff's opinion this movie is a total classic. How about Contact? A 7.3 on IMDb, not too bad, but still in any sci-fi buff's view this is a 8.0 minimum type movie. It is simply brilliant. Mission To Mars is another victim. It's a genuinely decent piece of sci-fi but in pop-culture, like Contact and Stargate, it gets ridiculed.
Average joes exit the cinema after seeing Prometheus and think "hang on that movie just tried to convince me that I was created by a extraterrestrial called an Engineer" quickly followed by "that simply isn't true!"
Whereas with Alien in 1979 they exited thinking "yes yes yes that really looks like how the future will be, and I'm sure somewhere out there a nasty alien like that really exists so that whole movie was quite plausible to happen in a couple centuries time."