I sometimes wonder if in some cases, it has to do with wearing such high heels and she is just uncomfortable.
But that's what makes it so clumsy and masculine-looking. To take long strides wearing heels, she either has to pick up her knees and put her feet down flat-footed, or she swings her leg, points her foot, and has to move VERY FAST with VERY LONG STRIDES to keep from falling over. The first method is a very stompy and arm-swinging stride, like a cartoon boxer. The other one is basically a goose-step, very artificial.
Interestingly, with both methods the upper body tends to tilt forward, the shoulders slumping and the butt sticking out. It's an accomplishment that Mary does this while holding her nose up!
This is probably a huge part of why Mary and Fred are never in synch when they walk together. Neither length of stride nor pace are in unison.
The smoothest way to walk in heels is to swing the leg forward from the hip and put the heel down first - but moving at a normal speed. Heel-toe stride, and the higher the heel, the shorter. It's a very flowing look and particularly graceful with a long skirt. Also, because the stride is shorter, usually the steps are quicker. Dancers know how to do this, and models. And actresses in old movies.