Casual audience members aren't people who follow the Tomatometer to the extent that they sign up for a movie based website, leaving reviews as though they're the next Gene Siskel.  Also, people don't have to hate a movie to acknowledge a lack in quality and realize it's not worth their money.  Regardless of Rotten Tomatoes user ratings, I find it much more telling that, after the "sub-par" X-Men movies, interest in the X-Men franchise dropped.
		
		
	 
I'm sure it has nothing to do with the fact that the series has overextended itself to the point of spin offs and prequels and is already 2 movies beyond a film called "
The Last Stand" that was meant to conclude the series in the GA's eyes, and is still constantly giving us the same story of humans hate mutants but mutants still fight to protect those that fear and hate them.
And that's not a knock on 
X-Men: First Class which was a great movie. But they really are rehashing the same storyline over and over again. I remember general audience complains when 
X-Men: The Last Stand came out that "Magneto is 
STILL the bad guy? When are they gonna do something new?"
Also, you can justify it how you want, but Rotten Tomatoes 
does have an audience review section, and it 
does get reviews, and both 
X-Men: The Last Stand and 
X-Men Origins: Wolverine, which the fanbase wants to blame for 
X-Men: First Class's poor box office performance, both have positive audience marks, indicating that the general audience did in fact enjoy those movies.
Those 2 movies aren't the reason for the disappointing performance of 
X-Men: First Class. I firmly believe the franchise has over-extended itself, thus the drop in interest. In the GA's eyes, these movies are just rehashing the same thing over and over again.
As a fan, 
X-Men: First Class was an amazing movie, because it showed 2 vital characters of the lore and showed us their history together, and it did it very well. As a general audience member, it's "oh, another one".