I never minded Megan as much as most fans. I know a lot of fans (here or TWOP? Unsure on the timeline) thought she was disingenuous for saying she wanted to learn about advertising when her dream was really to be an actress, but I thought that was fairly realistic. A lot of people have dream careers that they realize they can’t have and move on happily to a second, third, fourth choice. But then when she had a wealthy husband and didn’t have to worry about things like actually making a living, she decided to give her big dream a chance.
I think her character got worse as the series went on, but I think Don’s treatment of her did as well, and that was why.