White smoke from a lawn mowers exhaust is most commonly an indicator that the lawn mowers engine is burning oil.
The lawn mower could be burning oil because the oil got into the cylinders somehow such as through bad piston rings or somehow else.
Or your lawn mower could have a blown head gasket that could be another cause of a riding lawn mower that blows white smoke.
Or you could have overfilled the riding mowers engine with too much oil which causes the oil sump to overflow and then cause the engine to burn off the excess engine oil.
If your engine oil level is correct and you haven't tipped the engine over then you could have a blown head gasket on the engine or the piston rings could be bad as well.