Is it right to hate a game for it's ending? No, not really. I think gamers just get a bit bitter over it and it is the last impression many gamers get out of a game, thus leaving a bad memory of the whole game stuck with you.
In the case of Mass Effect 3 (and of course this was going to be a discussion focused on that game, what were you thinking?), rather snowballed. I still don't understand how, when a game is good up until the last five minutes, it can now be considered so horrible you'd never buy a game from that company again. It's like saying that 95% of your life was fine but, damn, you died wrong and you hate everything in your afterlife-state because of it, meaning that you'll never buy life again. Specifically, I think the ME3 ending backlash was only the second step of the Kübler-Ross model, aka the stages of grief, that many players went through. Really really.
I've disliked endings in the past, mostly due to disappointment in the game overall, yet I've never hated an ending. ACIII was the most recent one that disappointed me. Ironically, I thought that there should have been more of an outcry regarding the endings, with a lack of choice, than the one ME3 got but, hey, that's life. However, that doesn't negate a mostly positive experience I had before I played through the ending. How often do you get a game ending that blows you away? Does that mean that the other games, with average endings, are just "average" because the ending wasn't perfect? And what of the games with no ending, like sports, racing, and many strategy/sim games? Are they not games because they don't have an ending?
In the end, my opinion doesn't matter except to me. People will hate on things that I enjoy and I'll probably hate on things that they enjoy. No use arguing about it because that makes no one happy.