The NBA's leaguewide average is 45% from 2 and 35% from three. 75% from the line.
http://www.basketball-reference.com/leagues/NBA_stats.html
If you want to equivocate "average" to "good" that's your perogative, not mine. That said, that actually does Turner a bit of a favor, since his finishing ability makes up for his lackluster jump shooting (the same applies to Westbrook, actually).
What are the conditions for this, though? Does Joe Blow shooting 3/4 on 3's over the season count him as a 75% 3 point shooter?
Also, the fg% isn't a good measure for a good shooter, because people like Tyson Chandler and Tyler Zeller, who only shoot within 4 feet of the basket, skew the ratings. A better ratings system would only account for shots within certain ranges, i.e. mid-range shots, etc.
I'm well aware -- in fact, my second paragraph says exactly what your second paragraph says, which is why I brought in the shot charts in the first place. 
That wasn't my point. I differentiate between fg% and being a "good shooter." You might say it helps Turner since he's a decent finisher, but it still skews the whole percentage.
I don't think shots around the basket or one's ability to finish should define whether or not that person is a good shooter. So I highly doubt that the league average for actual mid-range jumpshots (again, excluding those closer shots and finishing ability that aren't necessarily relevant to the term good shooter) and longer shots is 45%, which is what my post was referring to. I'm not even sure where to find such information, but I'm fairly certain that it would make a difference when compared to pure fg%.
The shot charts account for all of this. Forgive me for being obvious, but average zones are white. The color moves toward red or green, for below average or above average respectively. Even though the league average for all 2 point shots including layups might be 45%, Turner's mid range zone is white at 41.9%. League average
from that zone is near 41.
Turner is, more or less, an average shooter with 2 horrible zones behind the 3 point arc. He is good from the corners and appears to be good when he gets to the left baseline, but these are small samples. I would expect every zone on Turner's chart to trend towards white as the season rolls on because... drumroll... Turner is an average shooter.