I'll give that a thumbs up to acknowledge the kernel of truth. Also, that last year's trends, and the years before that, don't guarantee a continuation. Nor am I suggesting he lacks mental fortitude or anything insulting like that.
But pitchers pitch differently under different circumstances. And Wallner's numbers suggest that he does less well when the pitcher's incentive is to use their best stuff (and/or the opposing manager hasn't yet put someone in with less impressive stuff). Certain fastballs, for instance, are said to give him more trouble than others (me, I'm not a good enough observer to have noticed the 4-seamers I recall being mentioned). If pitchers or managers ease off against him when the game isn't close, then I don't know whether to call that the batter's "fault," but I do hope that he can figure out how to make the stats line up better when the game still hangs in the balance.