Tee Oscar's ranks, out of all qualified hitters:
K% (lowest to highest; good to bad)
123/168
BB% (highest to lowest; good to bad)
121/168
SwStr% (lowest to highest; good to bad)
161/168
How much do these things matter? I summed z-scored for those three stats for all qualified hitters, with the bigger positive sum meaning that the player has a worse cumulative z-score. Here is the top 30 (the worst 30 at these things). Obviously this is stupid because using SwStr and K% mostly just double counts the one skill....
5.58 Lewis Brinson
5.12 Joey Gallo
4.71 Carlos Gomez
4.48 Javier Baez
4.04 Ian Desmond
3.98 Rafael Devers
3.97 Michael A. Taylor
3.86 JaCoby Jones
3.68 Chris Davis
3.65 Yoenis Cespedes
3.64 Kole Calhoun
3.53 Matt Olson
3.43 Scott Kingery
3.41 Giancarlo Stanton
3.30 Amed Rosario
3.23 Brandon Crawford
3.11 Matt Kemp
3.03 Khris Davis
2.90 Teoscar Hernandez (19th)
2.79 Adam Jones
2.75 J.D. Martinez
2.67 Domingo Santana
2.66 Yoan Moncada
2.62 Marcell Ozuna
2.59 Eddie Rosario
2.53 Nicholas Castellanos
2.44 Derek Dietrich
2.43 Cody Bellinger
2.17 Paul DeJong
2.12 Justin Upton
It's hard to really glean much from the list and Teoscar's placement on it. Some of the players are good most years because they simply crush baseballs. Some or good but not consistently, because of whiff issues and lack of walks. Some just are not good. I think the context is worth noting though. Guys like Castellanos and Eddie Rosario are decent players but really just average(ish) regulars, not stars. Teoscar needs to continue hitting the ball really hard to be a good regular, because the defense doesn't seem like an asset and it's hard to imagine the K and BB skills really improving significantly. The good news is that his hard hit rates have been elite in 2017 and 2018 so the JD Martinez ceiling is definitely achievable.
For fun, here are the "best" 20 from the z-scores, in reverse order. Most of these players are good or great, but a few of them aren't. The holy trinity of hitting really is Walks, Whiffs, and Barrels. Some day Vlad Jr. will be the kind of that game.
-2.70 Bryce Harper
-2.75 Buster Posey
-2.86 Freddie Freeman
-2.86 Justin Bour
-2.89 Ketel Marte
-2.99 DJ LeMahieu
-3.14 Carlos Santana
-3.20 Michael Brantley
-3.25 Brett Gardner
-3.27 Jose Martinez
-3.46 Mookie Betts
-3.68 Joey Votto
-3.93 Jesse Winker
-4.18 Nick Markakis
-4.35 Mike Trout
-4.39 Andrelton Simmons
-4.47 Denard Span
-4.66 Alex Bregman
-4.75 Jose Ramirez
-4.82 Joe Mauer