Most essential oils are too powerful to put directly on the skin, so in general, it is not recommended. And if the skin is inflamed, like a pimple, then it could cause quite a reaction.
Having said that, I usually put the tea tree directly on my pimples, and I don't suffer any negative reaction from it. But for maintenance, I rub it on the top side of my feet where the veins are close to the surface of the skin.
You could cut the Tea tree oil with jojoba oil to dilute it and apply that directly on the pimples, but then, your face gets really oily... which attracts dirt.
Hope this helps, and let us know how it works for you.