According to Daren Willman's excellent website baseballsavant.com (which I highly recommend all of our readers use regularly), Mookie Betts is 7th best in baseball in batted ball exit velocity (min. 20 ABs) at roughly 94.5 mph. Exit velocity is one of the key measures that talent evaluators in the game use in their player evaluation, and the fact that Betts' is so high is a very encouraging sign.
The problem for Betts so far is poor trajectories. He's hitting too many infield fly balls and not enough line drives. Betts' infield fly ball percentage is 19%, 28th highest in baseball and about 9 percentage points higher than the MLB average, which usually hovers around 10%. Infield fly balls are essentially strikeouts put into play and can really cripple a player's results on batted balls.
This is also why Betts' hard hit%, a stat that combines exit velocity with trajectory, is a poor 10% (MLB avg. is about 15%) despite the excellent exit velocity.
I'm no expert sabermetrician, just a humble fantasy owner, but the fact that he's striking the ball well enough to generate a high exit velocity but is hitting into poor trajectories makes me think he's just a little bit off. With a minor adjustment, either through natural progression with increased repetitions or a tangible tweak, strong outcomes could be right around the corner.