What does it mean if you find excessive burned oil on top of your piston when you go to rebuild a single cylinder two-stroke motocycle engine?

Either over oiling or a 'cold' spark plug. It is difficult to know as two strokes tend toward oily piston tops anyway. Mix-in-tank bikes are susceptible to over oiling, evaporation of the fuel increases the oil/ fuel ratio. Plugs have different temperature ranges. Changing the plug for a hotter one can often reduce fouling and improve performance. As will a properly baffled and clean muffler. Plug and points gaps and timing are critical.