Mercy has arguably the highest value ult of all the supports. Her ult causes game changing situations. Therefore, she is targeted far more than most heroes and has little ability to defend herself when she's trying to keep a teammate alive too. A good Mercy has to understand positioning, her guardian angel ability and teammate's positions, when to rez, and they need to be able to read a situation quickly to anticipate an enemy ult. While Zen and Lucio's ults are usually spur of the moment, done during an offensive ult to protect their team, a Mercy has to realize when a Pharah is beginning to put herself into position, or a Reaper is flanking looking for an opportunity. It is not that easy actually, especially when you have a long blazing line from your teammate to you showing exactly where you are.
But, no hero is easy to play. So please, get the idea out of your mind. Anyone can play any hero, but not everyone can play any hero WELL.
While it's true that most Mercy players who complain about dying are likely badly positioned, I have had enemies walk right past my allies to get to me as Mercy too. It can go either way but it's more important to let it go and keep your head in the game where it belongs. Getting salty benefits no hero or game.