So I started a little search. Kathryn really is not listed at the imdb page of Mistress America. But different reviews mentioned her name. She plays Tracy's Mom:
The movie premiered at the 2015 Sundance Film Festival on January 24, 2015.
On January 9, 2015, Fox Searchlight Pictures acquired worldwide distribution rights for the film
MISTRESS AMERICA was released in select theaters on Friday, August 14. But when will Noah Baumbach's hilarious new comedy be coming to your town? Check out our new Theaters Page on our official site to find out! View the trailer below to get excited.