As long as you are not exceeding the load rating of the dimmer, it would be normal. In service, it is normal for dimmers to feel warm to the touch: it is completely acceptable as long as the temperatures remain within the limits established by UL and the manufacturer. Underwriter's Laboratories (UL) has specified limiting parameters for this type of heat buildup. The average dimmer operates at approximately 140°F; UL allows a maximum temperature of 195°F. When dimmers are ganged together, each dimmer's load must be derated to offset the resulting heat build-up from the presence of several units in the same wallbox.