As far as I can tell from the picture, the amount of carbon build-up is not abnormal, but the softness/wetness of the carbon is. That is usually caused by oil consumption (due to worn rings): the oil does not burn well in the brief time available and carbons up everything, especially the muffler. There is usually some blue smoke if the oil consumption is bad.
If there is an abnormal amount of sludge in the crankcase that is probably caused either by blow-by (leaky rings) or some fault with the breather (crankcase ventilation system). Generally, oil sludge in crankcases is easily washed off by petrol, kerosene or white spirit (white gas, to an American; also known as naptha).
I personally do not use harsh chemicals to clean engine parts (the cleaning agents are prone to being hard to remove, thus causing corrosion/etching later). I don't believe I've ever heard of anyone using caustic chemicals.
