you're unavoidably going to have some unburnt oil in your exhaust. Richer oil mix and richer jetting will increase the amount of unburnt oil, and so can retarded timing or erratic spark. That said, it doesnt mean theres a proble, it's unavoidable. if your bike didnt have unburnt oil comin out, it'd probably be running too hot and oil lean.
Anyway, that gets trapped in your baffle and builds up and leaks out wherever it can, in your case the hole for the rivet.
you can bust off those rivets and repack thye exhaust packing. on race bikes folks recommend repacking as often as every 10 hours run time (on a moped that's like 100-300 miles) to mitigate the oil saturation, but i dont know anyone that does it more than annually, and I know basically nobody in mopeds who does it.
Swoop jordan let his get so saturated it started coking up and would occasionally start burning