OP, if mom made this as a “joke” trust me, she KNOWS, & feels hurt and offended by the jokes your fiancé has made also as she has been made aware of the shit talking by your darling beloved.

Be a stand up guy, don’t be manipulated by your fiancé, if your mom apologised, take it at that, don’t let her miss her sons wedding because your fiancé who also shit talked her didn’t want her there.

What about the times when your fiancé talked smack about mom, did you stand up for your mom or did you laugh & make it seem like it’s okay for your fiancé to do that because your mom wasn’t around.

Don’t let her manipulate you please. Yes it’s normal to set boundaries with parents, but this is extreme & it’s only favouring one person, your fiancé who is bullying everyone else.