D.T.
If the child is in the carseat correctly and the straps are snug like they are supposed to, there is NO WAY she can get the straps into her mouth!! Try pinching the straps - if there's enough slack that they can fold back onto themselves then they are WAY too loose.
They do sell terry cloth covers for carseat straps but they are to prevent the strap from rubbing against the neck -- because in a properly installed/used carseat the straps are snug against the child's chest/neck and can dig into the neck a little if the shirt doesn't have a collar.