Groucho Marx rarely mentioned his frequent co-star Margaret Dumont without claiming that she never got the jokes; that he'd hurl some ribald innuendo her way and then she'd come to him later and cluelessly ask, "Julius, what does that mean?" A number of folks like whoever posted as SisterCelluloid are questioning that view of her.
I think the piece overstates her importance to the act a bit when it says "This is a woman who was as responsible as anyone on earth for the success of his films" but she was very, very good with whatever they gave her to do, which often did not amount to a lot of screen time. Then again, she sure did work a lot for a couple of decades there and especially when she was on the stage with them, she had to have understood what the audience was laughing at.
As much as I admired Groucho Marx, he never seemed to be a good judge of women, especially with that last one he let into his life. I'm inclined to think Madame Dumont has been underestimated in the joke-understanding department.