No, and it could get you in trouble if you use one when you mean the other.
A mistress is someone that a person has an affair with when they are dating someone else or when they are married to someone else.
A lover is a romantic/sexual partner. A lover could be someone's wife, husband, girlfriend, boyfriend, someone they are having an affair with, or someone they just have sex with.
A wife or a girlfriend is a lover, but not a mistress. A mistress is a mistress and a lover.