She's not exceptionally good or bad either way. His Diana is very much the Proud Warrior Diana - which isn't a wrong take on her, that is a strong element of her personality, though the lack of the rest of her personality does cause him to completely fail at her friendship with Clark - and more than that, he seems to genuinely not disrespect her, at least not more so than he does any female character (he's never above an X-ray vision joke at her expense, but that's not about Diana specifically as a character). His contempt for Kyle just spills off the page, and he's actually pretty clever about getting in jokes or Tommy victories at Batman's expense that are totally fair and don't actually diminish Bruce's Batgoddery (and, I admit, are sort of satisfying for my schadenfruedey inner Bat-hate), but are nevertheless sort of mean-spirited when taken as a whole. Diana, though, is just a competent, together, heroic badass with a clearly-earned leadership position, neither worthy of fappery like Clark or debasement like Kyle, just a character.
Even his Wondy-expy in THE PRO wasn't particularly malicious. She was, in fact, one of the most unremarkable, together, and reasonably heroic characters in the book, with her only real played-up weirdness being her "all women are sisters" thing, which makes her seem alien and which the Pro rejects as inapplicable to herself - but "has an alien culture" is actually an accurate take on Diana. Compare to the totally incompetent and surprisingly malicious parody treatment the actual Diana got from the Superbuddies writers, for instance, who like heroes, and it's always been a bit baffling to me.
But if he respects the soldier- and warrior-type heroes, who don't agonize over snapping a neck what needs snapping, I can see how he'd be able to connect with that aspect of her character and therefore get her better and therefore treat her better than expected, and also, why he filters out the rest of her personality.