It really depends on the execution, personally I try to stay away from judgments about a work being pretentious unless I just genuinely can't think of a better substitute to describe the feeling though. But if I had to try and define it, I think it's a matter of some kind of implied self assuredness a work has in its intellect and merit in conveying an idea that in practice feels shallow, self-defeating, or vapid so as to feel wasteful. I feel like if a work is really preachy about an idea but fails to seriously consider the nuances or account for meaningful counterarguments, I might consider it as such because the laziness to not assess that assumed belief essentially serves to weaken the work and its message.

I'm not sure if this specifically is the work's fault or is a product of how some fans prop up this element, but as the closest concrete example I can think of: there's a certain shounen that comes to mind that gets hyped for its badass, independent, and relatively unsexualized female characters and I've seen people put down other shounens for the sake of propping up this one. In all fairness, there's a lot to criticize about how many popular battle shounens handle female characters and it's just a very common Jump thing for them to not be handled to the best they could, I can totally agree with that. And yet for whatever reason, the execution to me in this series specifically still wasn't satisfying. It felt more like a checklist of things people consider to be "good female character writing" on twt without the commitment to seriously and deeply explore them as individuals and people with character arcs or personal growth. Probably the biggest example was an episode cramming in the fights of almost all the relevant(ish) female characters of the arc, using one of said characters as a mouthpiece for the woes and stereotypes of how women are treated or have to act in the series without an actual meaningful or thoughtful response to it, and cramming it all into one episode with very limited payoff, all after a whole episode dedicated to the fight of a couple of male characters on the gimmicky side. This combined with one particular female character constantly being treated like a joke and made out to be useless (even in that very arc!) as a punchline left me deeply skeptical of what some fans had said, and some things I've heard about later developments with female characters doesn't inspire much confidence in me either.

Anyway, sorry for the rant, but I hope that kinda gives an idea. I don't want to slander the series discussed above (if you know, you know), but basically if a story feels assured in its merit in communicating an idea while lacking the substance to back it up, that kinda ends up as what I might deem pretentious. The above example might somewhat be me conflating that with (an admittedly very toxic subset of) fan statements and comparisons I don't think hold up, but yeah. I do try to avoid making that a focal point of my critique though because I'm not sure if it's my place to invalidate an author's experiences or values simply bc I don't vibe with the execution or vision, but I do think when a story or element gets propped up in discourse that deeply conflicts with my viewpoint it runs the risk of evoking a stronger reaction from me as with this one.