Um. I have been a Suns fan since 1976. JC in my screen name is the same as the JC in your post. I gotta be honest. Jerry Colangelo did not draft very well either. I cannot count how many horrible busts he drafted. I don't think any team drafts that well to be honest. Some teams are better at developing their draft talent and some teams are particularly horrible at the draft (the Knicks come to mind), but the draft is a crap shoot for the most part.
For every Steve Nash, or Michael Finley, there was an Armen Gillian or an Ed Pinckney.
Of course he didn't land every single pick... nobody does. But your memory of his successes may be failing you.
JC's notable draft successes from 1975 onward include Alvan Adams, Ricky Sobers, Walter Davis, Larry Nance, Jay Humphries, Dan Majerle, Jeff Hornacek, Cedric Ceballos, Michael Finley, Wesley Person, Oliver Miller, Richard Dumas, Steve Nash, Shawn Marion, Amare Stoudamire... and also, Jayson Williams and Stephen Jackson, neither of whom ever played for Phoenix, but had very successful NBA careers elsewhere. And as Phrazbit noted, many of those were late first round (and later) picks.
As for "horrible busts," I wouldn't necessarily count Gilliam who had a respectable if unremarkable career. Maybe Ed Pinckney, maybe Tim Perry... but they both brought high quality acquisitions in trades. William Bedford definitely was a bust, but ended up being traded for James Edwards, which wasn't bad. Most of the other less successful picks Jerry made were often late first rounders like David Thirdkill, Kenny Battle, Malcolm Mackey, and the like... mostly where the large percentage of all team's picks are failures.
I would put the drafting record of the Suns under Jerry up against most other teams during that period of time, particularly since they were rarely drafting early. In any case, it blows the 18 or so years of drafts under Sarver out of the water.