No! I would not recommend this for a couple of reasons. First, the heat from the dishwasher could warp the plastic pieces and make them unusable. Second, a loose Lego brick could get jammed in the dishwasher and damage the machine or get lost. Instead, I recommend filling your sink with some hot soapy water and letting your Lego pieces soak there to remove the paint (this is also where washable paint comes in handy).