How can I make a "lighter" cheese sauce?
I see two options for you.


The author of this Q How would I produce (stable) foamy bechamel sauce? reports success using gelatin. That Q is Bechamel sauce, which is the base of Mornay.


or


Use of an immersion blender to incorporate air into the sauce, making it a lighter texture. You can immersion blend at two points in the process: after the milk was thickened by the roux and/or after melting the cheese in. (I believe this is where @Pat Summer's answer was leading to)


(immersion blender)