It could be that both things are true, they did benefit from a weak East and they really are that good anyway. They are a well balanced, well coached team, they don't need heroic performances from their stars, everyone contributes and making it look easy. I'm surprised the Mavs haven't put more of a fight though. Before the Finals, it started looking like the Mavs were really coming together as team, but the Celts really picked that apart pretty easily.
Mavs have some work to do and something to build on, but they did seem to overachieve this season and didn't expect them to make it to the Finals. Luka has some work to do, for as brilliant as he is on offense, his defense is a liability and it was evident the way Boston attacked him.