In principle, sure. Lake Mead is in a warm enough location that it never freezes over (which could pose some difficulty for other lake-floated solar) and the intense heat and low humidity mean that quelling evaporation could be very beneficial.

In practice, the area of Lake Mead is about 250 square miles = ~160k acres * (~200 kW/acre) = ~30 GW of potential peak generating capacity. This means that a solar farm covering Lake Mead would immediately become the largest power plant in the world at peak output, surpassing the Three Gorges Dam (hydro) by almost 50%. It would be more than a quarter of all solar capacity existing in the United States as of 2022. So the overall electricity sector may not be ready for such an undertaking.

Partial cover is more feasible at this point, eventually extending to the entire lake, hopefully.