Lesley's mother, IIRC (haven't seen all of the episodes), never appears in the show, whenever she's shown with her family, it's always her father, and she doesn't mention her mother at all.
Why? Lesley never got to know her mother, they died right after she was born, due to childbirth complications that followed.
Mr. Smiles was heavily distraught upon hearing the news of his wife's passing, but when given Lesley and seeing her innocent, do-no-wrong look, knew he needed to push his emotions to the side and be there for the little bundle of joy he was given, which is why he's so silly and awkward at times with her, he wants to bond with the only family member he has, though like most adults, doesn't know how to PROPERLY bond with their kid, though Lesley doesn't mind, unlike her friends.
Lesley might even be her mother's name, in order to honor them.