Of the 292 players with 300+ MLB PAs in 2019-2020, Riley ranks #290 at making contact when swinging at pitches in the zone...73.7%.
The 2 players worse than him are:
Joey Gallo - quite possibly the most damaging contact in the game, and very selective to the point he has a 15% BB rate that gives him a good overall OBP.
Jason Castro - a catcher whose value is mostly tied to playing a premium defensive position and excellent framing.
Both of those guys swing and miss a lot, but compensate with a fairly low ~25% swing rate at pitches out of the zone.
Riley swings at ~40% of pitches out of the zone (though he has improved to ~30% in his small 2020 sample size), also nearly worst in MLB. Additionally, Riley doesn't do anywhere near as much damage as Gallo when he does make contact, and doesn't play a premium defensive position like Castro. So far, Riley chases bad pitches very often, can't make contact on many strikes, doesn't play a premium position, and does good-not-great damage on the contact he does make.
A close comp to him very recently is Michael Chavis of the Red Sox, who sports a 75% contact rate on pitches in the zone, and swings at ~35% of pitches out of the zone. He projects as something like a .240/.300/.450 hitter. I'm guessing he won't be holding a job in Boston for long with his 33% K rate...Riley's is 36% by the way.
The Braves probably need more than that out of 3B.