That amount of carbon is normal, nothing surprising there.
B.T.W., you mentioned getting wrong/misguided info off the internet, have you gone to ninja250.org for maintenance documentation? That site has a very comprehensive "Technical and Repair Info" section. I guarantee you wouldn't have gotten the plugs wrong if you were reading ninja250.org directions.
It's amazing how brittle the piston alloy is, isn't it? Aluminum is a strange metal, makes good pistons but also makes good solid rocket booster engine fuel.
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