It’s not necessary unless you’re concerned a chunk of ice should fall on it and damage the motor. If so, put a board, like a thick piece of plywood across and anchor it with rocks/cinder blocks, etc.
When it comes to air conditioner maintenance, At ProSolutions Inc., we think it is better to not protect it all round with a tarp/cover as you may get insects and mice showing up as residents. Or moisture may get in during the winter causing corrosion.
Blowing snow and rain will not cause problems for your outside unit when you are properly maintaining it throughout the year. So remember, when it comes to Air Conditioner Maintenance, it’s important to be on top of it!