In general more traffic = more money.
Oh - and to really make money you need to make sales too, not just adsense income. The nice thing about sales is that in many niches it can really become significant even with little traffic.
Back to adsense: With some keywords it does take less visitors to get the same amount, but they're generally also harder to get traffic for.
The trick is, always, to find a topic you can make good content for - that is unique enough, yet enough people are looking for it too. The reason you need good content is that only with links can you get the really good traffic (depending on competition online, but still). And you cannot create all those links.