The internet is an echo chamber for hate. If you go on the internet disappointed and a little pissed off about something, you will fall into the vicious circle of forum discussions where everything is picked apart and analysed frame by frame by like minded fans, and argued back and forth with those on the other side of the fence, over and over again. Your disappointment will fester until it turns to hate, then that hate will poison everything you once thought beautiful; and then... you are lost, nothing but a slave to the flame wars. The cycle cannot be broken.
Of course, there are also many people who genuinely dislike ME3 for reasons beyond just the ending. I know I for one was far more annoyed by the cut-down dialogue and the frankly shoddy amount of bugs the game had compared to its predecessors, than I ever was about the ending. I actually kind of liked the ideas behind the ending (making the culmination of Shepard's journey to find a solution to the rift between organic and synthetic life, something that had been a staple of the universe since day 1, rather than it being some bullshit about dark energy that had only been brought up in one mission in ME2 that nobody remembered). What let the ending down in my opinion was mostly just a rushed and ultimately poor execution of those ideas, which was largely fixed in the EC. Beyond that, anything else I didn't like is a difference in vision between me and the writers. It may not change the fact that I don't like it, but it doesn't make me feel like I was personally 'let down'.
The other stuff I mentioned though, now that does make me feel let down, because it's not a difference in vision, it's shoddy workmanship that has no excuse. The dialogue (and more specifically the dialogue wheel) is the cornerstone of Mass Effect. Who the hell asked for it to be cut down? As for the bugs. They had no less time to develop ME3 than it's predecessor. So how come ever since I got it in 2009 I've never found a single bug in my copy of ME2, and yet some parts of ME3 feel distinctly unfinished by comparison?
So in short, yes, in my opinion a lot of people got (and in some cases still are) way more butt hurt about ME3's ending than they should have done. However, I also think the people who claim that the game was absolutely perfect up until the last few minutes are blowing their side of the argument out of all proportion too. I love the game overall, but it does have its fair share of flaws other than the ending.