I think Karen embodies the spirit of Marilyn. I don't like Ivy at all. I think she looks horrible in the Marilyn makeup and hair. Megan Hilty looks much better with a lot less makeup. She has a good voice, but I never root for her or want to watch her the way I do with Kat.
For me, the actress playing Marilyn doesn't need to look exactly like her (and I don't think Megan does look that much like her - she's not nearly as beautiful). Getting a lookalike isn't the point. For me, it's about telling her story and getting an actress who is compelling and that people want to watch. I think Kat has that in spades. Obviously, Derek cares about that too. I knew Karen was going to get the part when he kept imagining her as Marilyn.
I think there might be some back and forth with the lead role next season, but it seems like they may be setting Ivy up for a downward spiral/cautionary tale storyline instead.