A quick glance at Mirotic's shooting stats from this season, and you can see the dreadfulness. He's currently shooting 27% from three, which is pretty hideous for anyone, let alone someone who is supposed to be regarded as the "stretch four" in Hoiberg's offense.
Another glance at his shot tracking shows that 77% of his shots come with 0 dribbles. The eye-test also confirms this. Niko is almost primarily being used as a spot-up 3-ball shooter. Mirotic has shown in the past that his game is more diverse than being used as a spot-up shooter.