It is never a good idea to mow the lawn when the grass is wet. Not only is this harmful to your mower, but it is also unhealthy for your grass. When the blades of grass are wet, they will stick together and clump up underneath your mower deck. This leads to dulling blades. Also wet grass tends to lay down, so you will miss patches of grass that need to be trimmed. If it is possible, do not mow the lawn during the hottest part of the day. Too much sun exposure on freshly cut grass and dry out and burn the crown system. Mowing at this time is also not very healthy or pleasant for the person doing the mowing.