Carbon deposits on the piston crown are normal. In mineral oil they are quite hard, but with synthetic oil they are soft and do not tend to grow much.
The carbon should not extend to the piston ring lands, where it will lead to ring sticking. Once the rings stick, exhaust gas blow-by will cause pistons to seize because the heat flow from piston to cylinder is disrupted.
When racing, I found some carbon on the piston skirt useful when running a mineral (dynosoar)/castor oil blend, just enough to fill in any irregularities so the piston could ride on a full oil film. Modern "synthetics" will clean very well, and prevent these deposits.
BTW,
Lawn boy is not an oil I would consider for my air cooled high performance engines, though it will probably perform well in lowly loaded weed cutters and chain saws. Ask professional lumberjacks what they use. Their saws have to work a lot harder, but then not quite as hard as our model airplane engines. A chainsaw engine has forced blower cooling and gets cooling pauses, which save their day.