Somewhat of a coincidence, yes. He was not responsible for Don Nelson, Dirk Nowitzki, Steve Nash, Michael Finley, the guys who built the current success. He's added something -- direction, passion, and throw in his deep pockets and there's a definite plus there. But its hardly an "appear in the NBA Finals" plus.
Big money teams have not consistently done that well in the NBA. Just ask the Knicks. You have to do some solid basketball work to make it ahppen. Meanwhile a traditionally fiscally responsible small market team like the Spurs has been there year after year.
The key fuss with Cuban is not his money, or his shaky acumen, its his complete and utter obnoxiousness. The epitome of a "new money" braggart who shoves it in your face. He's like the Rodney Dangerfield character in Caddyshack. No class whatsoever. Disrespectful. And borderline cheating too as he tries to pressure refs when he is effectively one of their bosses.