I always saw Madame F. as a sweet little old lady, about my own grandmother's age (mid-80s), who is a kid at heart, but has always been eccentric and as she's aged has become dreadfully out of touch with the rest of the world and reality. This was proven to me in the SW commentary.
I agree she was out of character in "The Big LaBlooski". At least all that yelling was. She never seemed to have much patience, though, as we see in "The Big Picture" when she won't allow Frankie to leave her side all-the-live-long-day long, and beating poor Mac over the head with her cane in "I Only have Surprise for You".
I think I mentioned this before in another thread, but my mother calls her "a lady after her own heart" after seeing "Say it isn't Sew". My mum LOVES sewing and crafting and quilting and such, and can stay all day long in a fabric store. She really liked that episode, and I thought it was funny I was getting my mother into the show, too.
