to console lennie over the death of his puppy, curley's wife tells him that he can get another one, that "the whole country is fulla mutts." what is the significance of that line?

This scene features some irony – Curley’s wife is not at all moved by the violence of Lennie’s action. Instead, she sees the pup as a replaceable object that shouldn’t be the cause of too much fretting. What she doesn’t see is that actually both she and the pup are objectified. The way she talks about the pup – violence against it is unimportant because the pup is considered unimportant – seems to be a bit of foreshadowing. When she is accidentally killed by Lennie’s violence, no one seems particularly sad that she’s gone; instead, they focus on Lennie.

I think this represents the how little life means. People don't amount to anything in Steinbeck's depression era novella. Men simply roam from farm to farm. Curley's wife could also be alluding to Curley being, like all men, nothing more than "mutts".

This can also refer to the life on a migrant worker, they can be easily replaced and the whole country is filled with them.