Does it have to be sarcastic? I was thinking "And I'm sorry for all the nasty things I said about your mother, even though we both know they're true." might be a case of Backhanded Apology
I think it can mean that the speaker (well, singer actually) is genuinely sorry about saying it, and is regretting vocalizing her thoughts about the listener's mother, while making it clear she still meant what she said. If that is the case, I suspect that it is not sarcastic.