And the tone of the ending is a tremendous problem. But the rest of the game is perfectly suitible in its tone.
As for half the galaxy being wiped out...pretty much every 'epic' story in existence has great battles in which countless innocents die. Where the good guys lose a great deal. It's just the nature of having a great evil in your story.
The tone of the rest of the game IS fine. IMO. The ending, was NOT. IMO. The problem that gives me is that it's dark and depressing piled on top of dark and depressing.
Yes, there are successes throughout the game - Hooray I brokered piece between the Geth and the Quarians - Oh sh*t, I just wiped out the Geth with the AI death beam.....Yippee, I've cured the Genophage - Oh sh*t I've just stranded Wrex in the Sol system. Thessia's free - Shame Liara will never see it again....etc etc.
I'm not criticising 'dark' storytelling - in many ways, I actually prefer it. Even in terms of the ending, I don't dislike it simply because it's a hollow victory, that's fine too. I saw ME3 as a long, hard slog - yes, punctuated, with moments of success, but generally, following a downward spiral into a finale that provided the merest uptick in tone. For me, that didn't work. The Citadel DLC, was tonally different, and was a tone that I welcomed having experienced the ending beforehand. Narratively, it makes little sense - hooray, a party while the rest of the galaxy burns, but I'd long since given up on the narrative making any sense at the time of playing the DLC.