Her singing doesn't have to be good, but it does have to be loud and boisterous. Helena's voice is super quiet and soft. She took a lot of the humor out of Mrs. Lovett's songs for me, so I'd be worried about the same thing happening with Mme. Thenardier.
Though, I mean, she isn't the role I am most concerned about. I'd rather they cast a bad singer as Mme. Thenardier than, say, Enjolras.
Jimmy, what are you doing here in the middle of the night? It's almost 9 PM!