Noooo - don't go feeding her. Let mum get on with it.
Size of udder or teats has nothing to do with amount of milk. Plenty of flat chested women successfully breast feed. She won't let you draw any milk. If the lamb is happy, she's getting what she needs, no reason on earth why there should be a problem.
Seriously I'd have concluded my first Shetland to lamb had no milk cos I couldn't draw any, wasnt feeding cos I never saw it happen, nor did I ever see the lamb poo. But the lamb was fine so clearly all those things were going on.
A hungry lamb is obvious. It cries lots, is obviously miserable and unsettled and is hunched up with an arched back and seriously concave (not the normal thinness of Shetland lambs) sides.
This lamb of yours looks fine, try to relax and let nature get on with raising lambs.