Molar enthalpy of vapourisation of a liquid is the amount of heat required to convert 1 mole of the liquid to gas/vapour.
Acetone does not consist of any hydrogen bond like water that is why the intermolecular attractive forces in acetone will be less that that of water molecules which makes acetone boil faster. Dipole-dipole interactions are there in acetone, which are relatively weaker.
So, due to this lesser energy will be required to convert 1 mol of liquid to vapour for acetone than water.
Hence, the molar enthalpy of vapourisation of acetone is less than that of water.