You can debate what 'good' is all you want, but pretty much EVERY NBA coach played at least college ball, most of them played in the NBA. We compare NBA players against other NBA players, so in that context we call some of them bad, but we all know even the 'bad' NBA players are very good basketball players.

Let's look at the winningest coaches in NBA history:
Greg Popovich- Played for Air Force Academy, team captain, leading scorer
Rick Adelman- WCC player of the year, 6 years in the NBA
Larry Brown- Played at UNC, Gold Medal in 1964, MVP of ABA All-Star game
George Karl- Played at UNC, Played for the Spurs for a few years
Phil Jackson- 6th man on 1973 NBA Champs Knicks. NBA starter after that
Pat Riley- Played at Kentucky, SEC Player of the year, AP Third Team All-American. Interesting side not... drafted by the Dallas Cowboys as a wide receiver. Key player on 1972 Lakers championship team
Jerry Sloan- The 'Original Bull' a 2x NBA All-Star, 4x NBA All-Defensive First Team
Lenny Wilkins- 13x NBA All-Star as a player.
Don Nelson- 2x All-American in college, NBA Champion with the Celtics, averaged more than 10ppg for 7 years considered one of the best 'sixth-men' to ever play.

What do all of these GREAT coaches have in common? They were good enough to play college ball, and all but Popovich (he was in the military) were good enough to play professionally. Being a good/great player does not MAKE you a good coach, but surely it HELPS.