Well I can understand the point of view, though I'm not 100% sold on it. Team sports are more than just about one person's production on the court. Our best teams were a testament to that - chemistry matters a lot. You often hear the notion that you don't have to get along with another to play together, but I'd counter that it definitely doesn't hurt, especially on such an unstable and dysfunctional team as ours.
It's no surprise that much talk revolves around something beyond on-court production. There are two issues that are being discussed: 1) Trading away a 27-10 player, and 2) The fact that it seems nobody valued said player enough to want to offer much. Of course, the simplest solution to these two issues is that Vlade is incompetent and got maybe 50% as good a deal as he should have. Most agree that is at least partially true. Nonetheless, it points to PERCEPTION of Cousins around the league (by coaches who haven't coached him, by GMs who haven't interacted with him) as not being that high. Everybody recognises his talent, but there's no way to know what is the truth as you have mentioned. Facts are: puts up big numbers on a bad team, clearly talented, one of best bigs in the league, hasn't won as franchise player, has had issues with refs, media and coaches, possibly team mates. Those are facts. Given those facts and the potential issues, good teams aren't willing to trade for him because it might cost them their team chemistry (e.g. Boston), can't just give up picks + filler because they would be late picks that Kings don't want. Other bad teams may not be willing to trade for him because he would just be in a similar losing situation as the franchise player.
I mean, look at all the talk of Carmelo being traded for the last x years. How much of the discussion has been about his actual individual basketball play?
Also, in a tight-knit league you're not going to get many players openly saying bad things about other players, ESPECIALLY young ones who are trying to find a place in the league. So yes, it is very rumor-based journalism, but I'm not sure what else you'd expect. Would you go on record to the media to complain about a colleague or boss, no matter how much you hated them?
All these talking heads are paid to talk. Most of them (and likewise us) have no real clue of who Cousins is, what he's like in the locker room. The stats are there, and everybody acknowledges it. People are saying that NO now can make a good run at the playoffs and maybe put up a fight against GS. That is basketball talk. But when it comes down to the trade itself, what else can you say that is basketball-related? The NBA is no different from the rest of life - it's far harder to rid yourself of a negative image.
Conclusion: I get where you're coming from, but I'm not sure what else you'd expect. If it were simply about basketball on-court, if it were simply about the numbers Cousins puts up, he either wouldn't be traded, or we'd be getting a much much better haul.