Trust me when I tell you that it is possible to feel a pang of barbed hurt when other people make light of your useless ex.
I’m not entirely sure why it happens, especially if the ex has been a particularly masterfully useless waste of space, it just does.
The best way to deal with exterior comments is to do your level best to let them wash over you. It was somebody else’s conversation after all, you just caught wind of it, you didn’t initiate it and you probably didn’t need reminding of how dreadful they were; presumably that was part of the reason you parted company.
So, flip the kettle on and go wash your hands of the words. Don’t engage in a response, it’s not worth it.