I have found some of my ponies love household food scraps and dog food and others won't go anywhere near it. All a matter of taste and how they were reared, they seek out the variety, doesn't mean she is lacking in anything nutritionally. Could also be a boredom thing too. As long as she isn't going to eat something she shouldn't (cigarette butts/chocolate) or cut herself on a tin can or something, I wouldn't worry too much.
I don't know about weightlifter being good for broodmares as it is formulated for underweight horses that you want to put on weight quickly, I would think any of the broodmare mixes would be much better because you know they are formulated for that exact purpose. You don't want to put too much weight on an in-foal broodmare as they can have birthing problems, but you do want your mares on an "upward plane of nutrition" when you are trying to get them in foal as it helps their fertility.