Heidi Foss who voices Mary Moo Cow as well as the woman that does is the actor under the costume in The Last of Mary Moo Cow episode, is primarilly a stand up comic in English Canada. Based in Montreal she is quite well known in the comedy standup circuit and Just for Laughs. I attach a link to her standup show. It cracks me up when I see her do these small parts on Arthur because her personna is so strong and such a funny mix of adult comedy mixed with a mainstream kid's show. I find her sense of humor very droll and also carefully not getting laughs at the expense of others which is kind of rare  since the days of  Johnny Carson or later Seinfeld. I attach a link to Heidi's show and Mary Moo Cow as well as the Boring Woman who is the animated  person who plays Mary when DW visits the studio.

http://www.comedynest.com/montreal-comedy/heidi-foss-june-28-29-30/