Why does my Magic Mouse stop working intermittently when connected to iMac?

If you have not already, work thru the steps in the following support article.

If you can't connect your Magic Keyboard, Magic Mouse, or Magic Trackpad to your Mac - Apple Support

Could be from a low mouse battery or the mouse is somehow failing.

One way to check that, would be to test a new or a different wireless mouse.

Might also be interference from other wireless and/or high speed USB devices.

Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

Welcome!

Is this an Apple mouse or an aftermarket one? If the latter, please tell us its make and model.

My experience with BlueTooth mice, keyboards and trackpads is that local radio interference is most often the culprit for symptoms you describe. See Apple's take on it; it's the best first step.

Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

I have very few disconnect issues at home where our wifi-enabled printers, router-modem, and phone base station are farther away and below my keyboard/mouse tray.

I volunteer at a local museum where the volunteers' desk is ground-zero for all the museum's tech. Wireless phone base station, a wifi printer, the modem/router, and the credit card machine are all within 2-3 feet of where I use my laptop with an Apple Mouse. Disconnects happen several time during a 4-hour shift.