I just went back and watched the scene. The squirrel part is so quick compared to the other rescues like the dog in that same scene.

The dog got a lot more direct attention, and that makes sense since a lot of people have or really like dogs. But I've never seen anything where the hero stops to save an animal nobody would ever even put in a scene.

It wasn't played up like a bit like the dog, and that made it feel like a more natural thing this Superman would do. It turned a throwaway gag into a bit of character development that in 3 seconds showed us this Superman would put himself on the line to save any life.