Modrons go past quirky, in my opinion. From everything I've ever seen and heard, when they show up the tone of the game doesn't just get lighter, it gets slapstick.
In theory, I know this. I've always kind of wanted to run something where I pit players against creatures that have the exact same game mechanic stats as modrons, but change the name and describe them as looking completely different than the modrons of the books. But even if that did work, I've changed them from really being modrons at that point. I've just found a really easy way to get rid of them, is all.
The thing is, you can rightly point out that anything in the game is what we make of it. But going against the grain of how the material is presented is work, especially if you want to go somewhere with something and they players already strongly see it in a different light, because you've got to "unteach" before you teach.
Reskin them, though, and maybe I could fear them they way I would the Borg and the Cybermen. Reskin them, though, and maybe I could fear them they way I would the Borg and the Cybermen.
Here's it the thing: I'm arrogant. It's one of my flaws. Ninety-five percent of the time when I look at something, my first thought is, "I could do better." The rest of the time it is, "I'm in awe." But as a result, good or bad, I've never met a game system that I didn't want to change from how it was presented. I hardly ever meet anything that I don't want to 'improve'.
So this problem you are talking about? It's everything for me. Modrons and Slaad are just examples of the general trend, but they aren't oultiers. It's not like I look at them and think, "Those are worse than normal." I think, "There are some good ideas here, but a lot of it is just junk... just like everything else."
I'm equally annoyed with say, the portrayal of goblins or kobolds or dragons or demons or well lots of things. Don't get me started on the drow, or elves in general. At best, these things are done in a way that is 'not wrong', but even then it is 'not what I want' (arrogance again). So in my opinion, everything needs redacting. I can't help it. I do it to everything. System rewrite. I have to put my stamp on it.
Thing is, IME, most DMs are like this. They have their own way. The real test of a system isn't, "Is this what I want?"; it's, "How much effort do I have to do to make this usable?"
Exactly. I agree, absolutely. It's just that in the case of the modrons, my answer to that last question could very well be "Too much". The reskinning of the look of the thing would, for me anyway, have to be extensive enough that I don't recognize them as modrons in a metagame fashion, or I'll just pop the aforementioned Disney cartoon mental image back in their place.
My other solution to the problem I have with the modrons that doesn't completely do away with them is to create something else that is the living embodiment of Law ("Archon" would've been a good name but they used it already for something else) but say that those creatures have created mechanical servitors -- the modrons, obviously -- to perform certain duties for them. In context of the rest of the conversation on this thread, it would then clearly they would be considered extraplanar constructs.
Stepping back from the specific cases of modrons and slaadi, I wonder if there is a more general question here:
What should we do with polarizing monsters?
Some monsters are loved and hated in equal measure. Keeping them as is caters to the people that love them. Changing them caters to the people that hate them. Either way, someone is going to be unhappy.
I think that, for monsters, the default option should be to keep them as is.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.