A "mutt," of course, is a mongrel dog, like the one Lennie has just killed. Curley's wife's whole statement is:
"Don't you worry none. He was jus' a mutt. You can get another one easy. The whole country is fulla mutts."
She doesn't mean anything but the literal truth. Mixed-breed dogs multiply freely in the country. Some manage to attach themselves to humans. Some turn wild. A lot of them get run over on the highways, either accidentally or deliberately. Others starve or get eaten by coyotes. Slim's dog Lulu had nine pups. Slim says he drowned four of them to make sure Lulu had enough milk for the remaining five. Now Lennie has killed one of those. What will happen to the other four is anybody's guess. Who cares? The whole country is full of mutts. The girl is only trying to console Lennie, but she doesn't really care about the little puppy he killed. She wants attention. She doesn't want to talk about mongrel dogs. She wants to talk about herself.
This dumb girl should have gotten a hint of warning from the fact that Lennie has just killed his own puppy by petting it too hard. The subject of the dead puppy and the possible consequences for Lennie brings the two lonely people closer together.
She moved closer to him and she spoke soothingly. "Don't you worry about talkin' to me. Listen to the guys yell out there. They got four dollars bet in that tenement. None of them ain't gonna leave till it's over."
By "tenement" she means "tournament." She is assuring Lennie that no one will see them together because they are all involved in a horseshoe tournament. This doesn't doesn't necessarily mean that she is propositioning Lennie. She only means that it is all right for them to be sitting there talking because no one will disturb them. There is no one to protect her, either. George is probably pitching horseshoes with the others and assumes that Lennie is playing with his puppy in the barn. The only other man not pitching horseshoes is Candy, who is old and only has one hand. He is the one who discovers the girl's body.