Yes, its consistently bad.
Minor PL, but my first GF (who turned out to be a crazy psycho) was a huge fan of this show, so I may have some sour grapes (and yes we were both adults at the time). It was also my first exposure to what we nowadays would call woke culture (thanks in part to said GF).
Kids Next Door was basically everything I didn't like about Chip n' Dale's Rescue Rangers except with few, if any, of the positives. The stakes could change on a dime. One episode Number One treats being kicked out of the KND as the worst event in the world, another he's willing to leave just because someone has a picture of his ass (which of course they show at the end because CN was run by pedos--and yes I do in fact remember people obsessing over that).
The characters had no consistency. One episode dude is autistically obsessed with following plans, the next episode he's all like "who needs plans when you've got these devices?"
Oh, and every episode was an obvious play on some movie, in ways that would even make Family Guy say "geez, dial it back a bit." Like I recall one episode being a Lord of the Rings parody except about flushing a dead goldfish down a drain, and for some reason they had Number Three acting like Gollum and Four acting like Frodo, bending them over backwards to make the parody work. This led to the universe being basically nonsense.
And none of the villains ever had actual plots. They all just had some power or ability that put everything on autopilot. Even in that fucking movie, the old shadow being just touches the ground and it sends out this wave that turns people old, sort of like that Ducktales episode with the golden goose except not treated as a natural disaster.
Doesn't help that the heroes are fucking assholes. Number One finds out the Delightful Children are actually an old KND team that got hit with a mind-control ray, and as said mind control is re-asserting control they give a heartfelt "we love them and we miss them" speech.... and One's only reaction is to be a dismissive asshole about it. Or how about the girls finding out that One is under the control of a "boyfriend helmet" that they KNOW is gonna eventually merge with his brain, this thing is apparently known in their universe.... and they don't care, at all. It's Two and Four who want to save him. Three and Five just go back to their comics.
This is before you get into the issue that a lot of the KND's own rules and inner workings are just... wrong to the point of almost being evil. They mind-wipe you as soon as you turn thirteen unless you're fit for service... and they're the good guys! Fuck that, I'd rather join the adults.
I recall hearing a rumor that Warburton actually lifted ideas from fanfiction. Most notably apparently the final episode reveals that Number Three was secretly always depressed but just hiding it (I was never autistic enough to read KND fanfic, but I had that psycho GF).
And yes, I know its spelled "Numbuh" and not "Number" but I refuse to honor this show's retarded baby spellings.
Which can we talk about how this show is a key example of "writers don't understand children?" The characters are supposed to be ten, and yet for some reason they act like they're five (including the retarded spellings). It also writes the kids as basically, little adults... including in romantic matters, when they're ten. I especially notice this with the higher-ups, one of whom I recall being this extreme career-minded woman.... she's ten.
To be fair, some of it seems like Warburton was writing basically a sort of 1950s idea of childhood. I couldn't help but notice you never see these kids playing video games or something except briefly, but they are said early on to have a one-a-month routine of going out and buying new comic books. Like, fine, Warburton was just writing the childhood he remembers.
In fact a fan theory I remember liking was that the whole show was in these kids' imaginations. I'd have been fine with that (indeed I kinda liked that pilot episode which implied they only think they're fighting tyranny when actually they're just causing a ruckus).... but then why is their imagination so boring, so full of flat shapes and muted colors and boring-as-fuck technology? When I was a kid, my imaginary base had an underground arcade and a secret entrance to what at the time was the best mall, and my cat could fire lasers out of her eyes. But see, my childhood was awesome.
And I know you shouldn't judge a show by its fanbase, but I'm just tossing it in there.... most fans I met back in the day turned out to be either literal children (understandibly these were the ones with the least amount of fucktardery) or adults who were crazy (and yes, in some cases legit pedos). don't take this the wrong way: I'm not accusing you of being crazy, a pedo, or a literal child, I'm just saying most of the fans I met were. And the adults in the fandom very much did try to inject politics. This was actually what led to my breakup with that crazy ex--she went on about how on this forum she and her friends were taking a stand against Bush and I said "you know most of these people are literal children and don't know what the fuck you're on about, right?"
As far as I'm concerned, Codename Kids Next Door is patient zero for the rot that afflicts western animation nowadays, and is pretty much the worst cartoon ever made just for that alone. The atrocious writing, horrible art, and barely-there soundtrack would be bad enough, but it being my first exposure to how icky internet fanbases can be is a cherry on top.