Too much makeup - usually refers to too much foundation, bronzer, blush, eyeshadow, eye pencil or mascara (caked on)... Lipstick also can make the whole look 'too much', but if there's a powerful lipstick color, then everything else should be subtle and have perfect skin. =)
The only thing that can overdo it is excessive eye shadow and bold lipstick... because these things are quite obviously different when its washed off... the right amount of make up should alter her natural appearence more than nessecary... here's A LITTLE FUN FACT : Did you know that the term cosmetics comes from the scientific term "cosmos" which is the process of creating order out of chaos? LOL !!!
I think it's too much when the makeup literally contorts their face into a different shape with different shading and "fake-made-real-by-makeup" shadows and cheekbones. I've seen those tutorial pictures where the women literally "change" the shape of their face (optical illusion). To me, that's too much. If women want to wear big bold eye makeup and dark red lipstick, I could care less really.
But once you're changing the whole outlook of your face...