Are medium cushion ski socks too thick for properly fitted boots?

No — for the majority of properly fitted ski boots, medium cushion is the ideal thickness. A proper boot fit should have minimal excess room but shouldn't be so tight that a medium cushion sock causes discomfort. If medium cushion socks make your boots feel too tight, your boots may be sized too small or too aggressively. Light cushion would be a better choice for very snug performance fits, but this represents a minority of recreational skiers.