On my laptops I use black electrical tape, much more stylish 😛

It used to be my job to spend all day trying to break in and exfiltrate data from secure systems. It's absolutely possible for your webcam to be turned on/accessed remotely, especially considering the average home user is generally terrible at keeping their systems up to date with security patches (yes, even Macs are vulnerable) I don't think it's terribly common though, most people who are going to do shit like that is bored kids and creeps, I'd worry a lot more about account numbers and passwords and shit other than access to the camera