She's a supporting character in more ways then just the technical term. It's true, shes either helping someone else, having them do work around Hinata, or she's busy. She's always keeping herself busy in some way or another when she's not managing the tea house, and we only really see her by herself on a handful of occasions or resting by herself less than that. At least in the anime that is. Of course the manga is more indepth and goes into her character a bit more. Thankfully.
To call Haruka the 'mother figure' would be an overstatement, but she does do more than her fair share in keeping Hinata Sou on track. We just never see these things because she's regarded as a 'supporting character' or 'minor character'. I would call Haitani and Shirai minor characters, since they only appear a handful of times or at key moments. Haruka is constantly there, whether participating in full, or doing her own thing in the background. I don't think it's fair to group her in the same way, therefore, as you would other less active characters.
Inevitably because of grouping and an underplay of her character she isn't as popular as the other characters. It's a real shame when this happens in anime, and it happens all the time. Maya Ibuki from Evangelion is an example I've used before, as is Julia in Cowboy Bebop.
