Lum's mother is different from Ataru's mother. First of all, you can't understand her. The official website of the series states "The only thing you can hear from her is a jibberish-like Oni language often depicted as mah-jong tiles in the manga." The book, The Return of Lum Sweet Revenge, shows her talking. Also, she looks similar to Lum except a little older.
Lum's mother shows up for Parents' Day (which is when the mothers spend the day at school watching their children). She lands a spaceship on top of a oxen-drawn wagon that had Mendo's mother. Mendo is a student at Tomobiki School, and is in the same classroom as Lum and Ataru.
Translation: "Thank you for destroying our carriage!"
Mendo: "The enemy of my carriage is my enemy!"
Translation: "My carriage is my enemy!"
Mendo: "Someday I will demand payment!"
Translation: "Someday I'll repay you!"
Mendo's mother flipped a glove at her, and she doesn't understand. The reason Mendo's mother does this is because Lum's mother was hugging Mendo. When Lum's mother learns that it means that she is being challenged to a duel, Lum translates:
"Madam, please reconsider! We are both women! To do a thing like this...I'm very flattered that you'd ask me to marry you, but... regretfully, I must decline!"