I love Nicole, and she is a great actress. However, I think she was miscast, and on top of that, the makeup people let her down, in my opinion. She just did not LOOK enough like the real Lucy to make me believe it, and as good as she was, her acting just could not overcome that. For me.
An example of the makeup department doing their job is the current transformation of Lily James into Pamela Anderson on Hulu. That is phenomenal! She spent 4 hours every day in makeup. They gave her prosthetics (boobs, forehead, false teeth to look like Pam's and give her lips the right shape) and wigs. And as a result, Lily is pulling it off. (So is Sebastian Stan as Tommy Lee. He went through 4 hours of prosthetics and tattoo applications every day.) I was never a fan of either Pam or Tommy, and I didn't pay much attention to their sex tape scandal, but these performances have captivated me! (Seth Rogen is awesome, too, but he didn't need special makeup.) That is a makeup department doing their job!
Anyway, even something as simple as redrawing Nicole's lipline to match Lucy's would have helped for me. This was about recreating an icon. I missed those lips every time I saw her and that reminded me it wasn't really Lucy enough right there to boot me out of my immersion in the story. It had nothing to do with Nicole's acting or whether or not she portrayed the "real" Lucy vs her TV character. It just did not work for me. I simply could not BELIEVE it, and that's an essential ingredient. Not even Meryl Streep could have pulled that off without some help! IMHO.