Yes... and no.. technically they are. While MIPS uses a slip plane, it is not going to roll around all willy nilly with every head movement. It takes a lot of force to get it to move, as it is suppose to. Nor should you ride while making bobble-head like movements anyway :P
Many new helmets come with a either built in or removable camera/light mount that have MIPS technology... The Giro Montaro for example is a MIPS helmet that boasts full accessory compatibility as well as many other that do.
The kicker is - Having a camera or light mounted may prevent the MIPS from doing what it is suppose to.
Vents are necessary, but make sure they are smoothly faired into the helmet shell, and avoid any helmet with unnecessary fashion ridges on the outside, or snaps for visors, or any other feature that could cause the shell to snag. This is an easy item for a consumer to assess, as long as you keep in mind that you want your head to slide on impact. It should be evident that you don't want to add any accessory or cover to the exterior of a helmet that adds to its sliding resistance. That includes lights and cameras. Many of the ones we see have mounts that are much too strong to break away easily when you need to slide. ASTM is developing a standard for the force that should trigger an accessory to break away. Bell already has its own internal standard. In the meantime you are on your own to figure it out.
Sourced from: BHSI
Basically if you don't want to read that, mips prevents your head from catching and rolling your neck in layman's terms. So that the helmet rotates rather than you neck. Having a large camera or light on top can catch on the ground and prevent it from doing what it is made to do. So while they are advertised as being compatible, what they don't tell you is in the event of a crash, accessories may be what causes injury and prevents the MIPS from moving correctly.
Also that link is a good source of information on MIPS technology in general and points out a lot of flaws with the system. A worth while read if you have a few minutes.