I've been paying attention to the insults that Carla hurls at people. Although no one is off limits to her biting sarcasm, she's more mean-spirited towards certain characters. I think she can be merciless to Cliff, Diane, Frasier and Rebecca. She goes easier on Woody, and is on the nice side towards Norm. Observing her with Norm, she often appears to be quite fond of him. I think she was extra nice to Coach.
It's obvious how Carla feels about her *Sammy*-- she considers him a very good friend, and her sarcasm doesn't have that below-the-belt tone with him.
When all is said and done though, I think Carla genuinely cared for everyone at Cheers, and that was shown to viewers from time to time.
The acid-tongue woman was really a softie at heart.

my favorite carla insult is

Cliff: I spent most of my young days in a laboratory

Carla: and you'd still be there if the chimpanzee didn't show you how to open your cage

I also noticed that she never insulted Sam or spoke mean to him. Well, he was her boss of course but I think she actually respected him as a friend. :)