We have not been to Winter/Summerland, but were to The Fantasia mini golf course, which was totally accessible. Winter/Summerland was built later, so it should be at least (if not more) accessible.
As BCV23 mentioned, there are "curbs" to keep the balls contained, but there is a space where you can enter the curbed area with a wheelchair and there are no steps in the "golfing area."