Kazuki is fine.
Arguably, he's a better romantic co-lead than the usual fare for these titles, wherein he'd be just another nondescript bland "background character" who somehow pulls the hot perfect idol babe because he was nice to her once when they were kids and she never forgot it while he goes most of the story without knowing they have history. Storywriting-wise, he's leaps and bounds more interesting as a character.
As a
person, he's just a moody teenager who would arguably be a bit exhausting to put up with (look at his mother for reference), but even then we have a solid "now vs. then" motif showing who he was when younger, a hint as to what happened to make him the shut-in, avoidant person he is at present, and we have a path toward him growing past this attitude as he gets over himself (likely with the help of Sayaka).
The only thing that would ruin this is if the author doesn't have Kazuki meaningfully confront himself and the way he's become and make changes to himself. If he stays this lazy, insular & avoidant person and still gets "rewarded" with Sayaka as a girlfriend without addressing the obvious character flaws he's been presented with, then I think it's fair to criticize his character and the author's handling of him.
But we're nowhere near the point where that could be assumed to be the case, so it's not worth thinking about, here and now.