I’ve wondered about this for a long time. What did the teen hooker in the 80s film Mona Lisa mean by telling Bob Hoskins character (an ex con turned driver for a prostitute. Of course he falls in love) that they can only eat ice cream? He asks but the only answer forthcoming is a smug smile at his naivete. Too many blow jobs was all I could figure, but then why was that only the case for the young girls and not the working gal he carried a torch for? I wonder about this any time another hooker story comes on TV. I’ve never seen anything else like this in any other story, teens or not.
Isn’t there something about junkies and ice cream? Drug use among street prostitutes is rife. Don’t know if that makes sense in the context of the movie as I don’t remember much about it.
Maybe because she was too young to legally be served alcohol, so he couldn’t take her out for a drink? They could only go to the proverbial “malt shop” instead? 
I must admit, though, I haven’t seen the film, so that’s just a WAG.
The main character had spent a long sentence in prison before taking on the driver job for his old crime boss. I thought it was weird he wouldn’t know. Junkies do love sugar. Maybe that’s it. I guess it was just a pointless scene.
I haven’t seen the film either, but it sounds like something to emphasise her youth, in contrast to her very ‘adult’ profession.
Or she might be a toothless crack-whore who can eat little else.
I do not kow the exact quote from the film, but I am currently reading The Gangs of New York, which has a mention near the end of Chapter XIV to “ice cream eaters” as being users of cocaine, heroin or morphine who chew the crystals instead of injecting or inhaling them. I take nothing in this book as valid or supported, but perhaps the write of the film was referencing a (currently) outdated druggie term.
That’s sorta it. Heroin has a reputation for depressing appetite in many addicts and fucking with the GI tract, but at the same time causing sugar cravings in some. Combine the two ( and that character was definitely an addict ) and you get that scene. She can’t bring herself to eat anything but ice cream anymore.