"When did morris dancing become a joke?"

Wasn't aware it had. I suspect there are degrees of ability/art/skill/respect for the tradition, and troupes that practise lots will have a better handle on the form than troupes that don't. I'd also suspect that some use it as an excuse to get shit-faced. They'd be the ones that make it a 'joke' I'd think. IMO--with no offense intended to anyone anywhere in the whole wide world.