The rain can definitely help improve the fishing. When you have big rains, it can make the water murky and sometimes, the fishing can be tougher because of this. On some lakes, there will be a dam or spillway attached and the heavy rains can sometimes turn the spillways into awesome fishing spots. You may be able to catch some muskie above and below these areas.
If you aren’t fishing dams and spillways and you get a light rain, the fishing can be awesome. There is something about a light rain that just keeps the fish active throughout the entire day, which gives you a lot more opportunities to get your lure in front of a feeding muskie. Heavy rains make fishing a little tougher, but that’s not because they won’t bite. It just makes it tougher on the angler.