I don't remember her either so checked the script.
http://scifiscripts.name2host.com/msol/manonthemoon.txt
I don't remember this scene, I think it might have been cut and the story line ammalgamated into the Lynne Margulies (Coutrney Love) character.
Well, that's my theory anyway.
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I was just watching the version of this on Netflix streaming and it seems at various points that they must have moved the scene (with respect to the script referenced above) and then deleted it.
The script has Mimi's scene between those showing the making of Andy's TV special (vertical hold argument) and the network execs discussing it. They probably thought it killed the pacing to place an unrelated scene between those two.
Reiko's placement in the credits (supposedly in order of appearance) would indicate that her appearance came between the scene of Tony's show at Harrah's and the Merv Griffin Show scene, or in one of them. Nothing remotely like the Mimi character in the script exists here in the cut of the movie I saw.
Unless Kaufman proposed marriage to women he hardly knew on multiple occasions, it does look like they took that bit from the Mimi scene and added it to his first date with Lynne, dropping the rest of the Mimi scene.