I think that you're right, but I also think there's a bit more to it.
Avengers does not take itself as seriously. It's primarily a comedy/adventure, and thus it makes sense that a lot of audience members don't feel sorry for the 100,000 fictional deaths in the movie, you need to make the effort to get people to care, and Avengers doesn't make that effort. These are fictional deaths after all, it's irrational to remotely care.
Man of Steel, on the other hand, does take itself seriously. Very Seriously. It asks, what if Superman was part of our world, and what if Zod invaded and wiped out a quarter of Metropolis? The creative team may have failed at other things, but I think here they succeed, I, and many other audience members, actually did relate to the fictional deaths. I did think "Oh no!" Snyder has stated in interviews that this was an actual goal, and they succeeded. Unlike in the Avengers, the destruction feels real. Snyder was successful in this.
Which thus makes it a lot worse that Superman and Lois decided to make out while being in a sea of death... I spent a month in a palliative unit of a hospital once, and trust me, making out was not my first priority. It also makes it worse that there was no acknowledgment of all the destruction, and that the movie ends on a happy ending with people joking around.
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Aside,
- Lois first learns of a Superman when she is almost dying after being attacked by a sentinel;
- Lois first learns that Superman is Clark when they meet at a cemetary;
- Lois and Clark first kiss when they are surrounded by death and destruction in every direction;

