MacBook Air M1 battery drops quickly

This might be normal behavior, as macOS can speed up the discharge rate when the battery is low to help optimize its lifespan. To get a clearer picture, check the battery health in System Preferences > Battery. You can also open Activity Monitor to see if any apps or processes are using more power than usual.

Most likely yes. Those things are destructive in nature.

"CleanMyMac" is not the only example; it's just popular. Definitely uninstall it and see what happens, however, from personal experience I can tell you uninstalling it (or things like it) will not undo the destruction they are capable of inflicting on a Mac. Completely erasing the Mac and reconfiguring it from the ground up is generally required.

Why? Read Is Clean My Mac a recommended program? - Apple Community. That link should take you right to the relevant post, but I'll copy an excerpt here for reference:

Case in point: a colleague brought a Mac to me that didn't seem as responsive as it should have been. Upon questioning her she admitted using CMM to "clean" it, but all remnants of the program had been removed. Yet it still performed poorly. Booted Safe Mode, no help. Still not running well. Fan blowing, etc. I could find no obvious culprit. Using Console (which I do not generally recommend) the problem turned out to be a macOS process that was encountering constant I/O errors writing to a file. It turned out to be a log file that CMM "cleaned". File no longer existed.

What could go wrong with deleting a simple log file? Apparently, a lot.

Log files are created and maintained by the system. macOS expects them to be present. Perhaps Apple's engineers should have foreseen the possibility of users sticking their fingers where they shouldn't be stuck, and anticipated the need to create a required file that mysteriously vanished. But at some point it's unreasonable to anticipate every possible dumb thing some malicious or merely inept developer might do. This program met all of Apple's App Store requirements. It was not "harmful" yet it still caused a perfectly good Mac to perform poorly.

This is also the reason I advocate erasing any Mac that has ever had any such "cleaning" product installed. It saves time. Instructing someone in Console over a support site like this would be an exercise in futility.

For the Mac in question, I created an empty log file where it should be and all was well.

Bottom line: Be prepared to erase that Mac.

... CleanMyMac app showing all is good.

"CleanMyMac" claims another victim.