Devers has made 20 errors in 19 games, of which the Red Sox have a 12-7 record (~102 wins for a season). As best I could determine with a quick look, his errors only accounted for unearned runs in four of those games (three of which were losses but not necessarily due to his errors).
Twenty errors is a lot for the number of games he has played but most of the damages caused by them has been extending innings and pitch counts.
Yes, but that just means that he's been lucky, or maybe more accurately, the Sox have been lucky where his errors are concerned. MLB-wide, each error is worth roughly half a run allowed (exactly 1000 unearned runs so far this year, resulting from 1848 errors).
OTOH, we need to remember how young he is. Since 1947 there have been just 13 21-year-old third basemen (70% or more of starts at 3B) with enough PA to qualify for the batting title. Here's how many errors they each made, with their fielding percentage:
Larry Parrish: 35 (.919)
Ron Santo: 31 (.937)
Eddie Mathews: 30 (.939)
Rafael Devers (pace): 30 (.929)
Gary Sheffield: 25 (.934)
Aurelio Rodriguez: 24 (.954)
Adrian Beltre: 23 (.944)
Buddy Bell: 22 (.958)
Brooks Robinson: 21 (.953)
Richie Hebner: 19 (.944)
Bob Bailey: 18 (.943)
Ryan Zimmerman: 15 (.965)
Eddie Yost: 15 (.966)
First, let's pause to notice the company he's in.
Second, while he's near the bottom of the group in both raw number of errors and fielding percentage, he's nothing like an outlier. He's within 8 percentage points of a five-time-Gold-Glove-winning Hall of Famer, and within 15 percentage points of a guy who some claim deserves consideration as the best 3B defender of all time.
In short: 3B is hard. 21-year-olds are unlikely to play it flawlessly. Give him time.