I think they can matter but not at face value. Veteran pitchers are going to be tuning their pitches more than trying to get guys out. But young players without a guaranteed spot will at least be trying to showcase their skills and make the team.
Stowers' 7 HRs are somewhat relevant in my eyes. That is such an outlier number that I imagine it has to move the needle a bit, especially with two in the last game being against a big league veteran.
I don't put much stock in Jackson Holliday's spring training OPS. He got a gift triple, and his one success vs a lefty was against the third fastball in a row from one of said veteran pitchers who was probably just working on his fastball that inning, so I think it's probably a bit inflated. Though for all I know, he got unlucky in the ABs that didn't make the news.
I imagine the front office has access to more useful numbers than what we see, taking into account the pitchers they faced, the set of pitches they faced, etc. Those numbers are more likely to be used to make a roster decision.