No, we've not yet seen Jayne's mother in any of the extended material. There's a mention of her in "The Message" when we see the awesome hat that she's made Jayne.
JAYNE : Haw haw. It's from my mother..."My dear boy. I hope you are well and that you get this soon in your travels." Travels..."Thank
you for the credits you forwarded, they have helped as Matty is still
sick with the Damp-lung. I made you the enclosed" --Ooh! Enclosed!
He reaches in and pulls out a woolly knitted hat with earflaps and a
pom pom. He is clearly moved. He puts it on, continues reading.
"enclosed to keep you warm in your travels. Hope to hear from you soon, love, your mother."
He closes the letter, proudly adjusting his hat.
There's also a further mention in the comic "Serenity : Better Days : Part 2" where we learn (in Jayne's fantasy story) that her name is Radiant Cobb.