Stewart was speaking Tuesday in Beverly Hills at Vanity Fair's 6th Annual New Establishment Summit at Wallis Annenberg Center for the Performing Arts.
During a "Discovery Gets Cooking" panel with CEO David Zaslav and CNN correspondent Brian Stelter, Stewart was asked if she had any advice for the "Deseparate Housewives" star during her 2-week prison stint.
"She should style her outfit a little bit more. She looked pretty schlumpy,” Stewart responded.
For those who didn't grow up with Jewish grandparents speaking Yiddish, "schlumpy" is used to describe a sloppy person.
Huffman was seen over the weekend wearing a green jumpsuit at FCI Dublin, where she is serving her sentence, and was visited by husband William H. Macy.
Stewart followed up that Huffman "made a horrible mistake, and she's experiencing what happens."
The star knows a thing or two about prison life, as the home & garden guru served 5 months at a federal womens camp in West Virginia for insider trading in 2004.
However, when Stewart was in prison she stayed true to her roots and regularly foraged for dandelions around camp for use in prison-style microwave recipes.
She also gained the admiration of her fellow inmates, and one woman even knitted Stewart a shawl to wear upon her release.
Huffman, who pled guilty to conspiracy to commit mail fraud and honest services mail fraud, probably won't have enough time to develop the same type of relationships with her bunkmates.
However, Lori Loughlin is looking like she will have plenty of time to relax in prison if prosecutors get their way.