I've been to WDW many times over Memorial Day weekend and I've never found it to be terribly crowded, except in DHS over the SW weekend. Monday should be fine. We usually spend Monday morning in the MK, and it's almost empty (partly bc it's usually open super late the night before).
Last summer I went June 7th-12th instead of Memorial Day weekend and June was MUCH busier than Memorial Day weekend. Even though I can only go Thursday night-Monday night, I'll stick with Memorial Day weekend from now on instead of June. FWIW, I always find Monday to be the last crowded day of the "weekend" because people like me have to fly home that day/night