White smoke coming out of your lawn mowers exhaust when running is an indication that your lawn mowers engine is burning oil.
The cause of the burning oil could be as simple as overfilling the engine oil or could be a bad crankcase filter or bad head gasket on the lawn mower.
I would first check the oil level and make sure it's not overfilled and if it is overfilled then you need to drain down the oil because it can cause engine damage if not drained down.
If it's the head gasket then it needs replaced or it could be caused by oil getting into the carburetor because the lawn mower was tipped over on it's side.