A quick calculation of what it would take to replace the dam with solar panels:

According to wikipedia the dam produces 1052MW. Let's call it 1GW. https://en.wikipedia.org/wiki/Mosul_Dam

According to quora you need about 10000m2 or one 1ha to produce 1MW. So 1000 * 10000m2 = 10km2 to produce 1GW.

I wonder how big the lake is? Could solar panels installed in the drained lake - theoretically - provide the same or more energy than the dam?

Of course energy storage is a major part of water power and it is not taken into account above. On the other hand a decentralized power grid is one of the major advantages of solar (especially in an unstable country) and that is not taken into account either.