Another suggestion from Reddit–if ever I run across a riff that no amount of research can turn up an answer for, I’ll post it here. If you happen to be reading and are able to answer it, please chime in.
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UNEXPLAINED RIFF: In the very beginning shot of “The Chicken of Tomorrow,” the short in front of ep. 702 “The Brute Man,” Mike comments that “the chickens are dining in front of a Susan Rothstein mural.” Crow laughs at this, but for the life of me I haven’t been able to dredge up anything on who “Susan Rothstein” is.
MST3k Riff-a-Day 
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Hello – the only reference I can find that may relate is there is a Susan Rothstein who once was the Executive Director of the Museum of Children’s Art in Oakland,CA:
http://www.lead411.com/company_MuseumofChildrensArt_1200191.html
Just a guess, though.
I thInk he meant Susan Rothenberg, an abstract painter.
Based on what little you can see of the “mural” and Susan Rothenburg’s style, I agree 100%. Apparently Mike was WINGING it that day! 😉