I don't want to post too many times, but I really would like to respond to this.
I don't think there is any type that's simply bad "in and of itself". Whether a Pokemon's type is good or bad for it is going to depend on what roles that Pokemon is trying to accomplish and then how that typing meshes with that role.
First and foremost, I would like to say I agree that no type is truly bad. Every type has both upsides and downsides. However, some are clearly better than others, and since this concept is supposed to involve "bad" typings, it is important that we can at least decide which types are comparatively bad.
For a Pokemon attempting to cover an offensive role and not a defensive role, Poison typing could surely be said to be bad, because it has so little to offer offensively (can't hit Steel, on SE vs Grass, etc.). On the other hand, for a Pokemon attempting to cover a defensive role and not an offensive role, then Poison typing wouldn't be considered bad, because it does offer defensive assets (resists Fighting, can't be poisoned, etc.). That's the thing about this concept that I think is so interesting. It's not just about the typing itself but rather what's being done with it.
I would tend to disagree with this. See, poison is not a bad typing at all. And one of my points I was trying to make is that a typing can only be bad if it is bad defensively. Sure poison might not have a great STAB, but its good defensive typing lets it get in, take a hit, and boost or sweep perfectly fine. It might not have good STAB coverage, but that is about the only problem it has. And, when you realize it has respectable neutral coverage which can be easily helped by just a single coverage move, you realize that poison is not bad offensively at all, its just that such a Pokemon doesn't exist yet.
However, with a typing that is very weak defensively, you can't even run many offensive sets well, because it dies too easily. When anything that is outspeeding you can kill you, if you are not fast as hell then you have a hard time fitting in, no matter how good your offensive prowess is. Types like this are really the only thing that can come even close to being called "bad".
I don't think that's true at all, I don't think that there's any better evidence of that than the result of Break the Mold: Strategem. Strategem would not be a good example of this concept at all, because it put a Rock-type Pokemon in an offensive role and not a defensive role, when Rock-type Pokemon are good at being in offensive roles and not good at being in defensive role. What Break the Mold did was on the level of flavor and stereotypes (perhaps on the level of mechanics as for as typical stats were concerned), while this concept that we have now can be said to be similar but is actually on the level of mechanics instead (as in, typing mechanics themselves).
Ok, now when I was referring to Break the Mold, I was referring to it being done on a proper competitive scale, not to a flavor based concept. If you think the actual Break the Mold concept is purely flavor, then that is your opinion, but that is not what I am referring to. I cant speak to how it was actually decided on with Stratagem, but honestly that doesn't matter. Strata fits both ways. It was not about offensive vs defensive, it is the fact that it was a Special Attacking Rock type that broke the mold. The fact is, Break the Mold was putting a typing in a role it was not used to. That didn't make it bad at it, just made it different. That is exactly the case with mono types for this concept. No mono type is "bad", and all we would be doing with it would be "breaking the mold." We don't have to put a type in a role it is not familiar with for this concept. What we have to do is take a type with glaring flaws and make it like its typing anyways. Nowhere in that concept does that say anything about trying to make a type work in a way it is not good at.
This is why, as cool as it would be to actually do, I'm not in favor of the offensive Ice/Rock Pokemon that's getting discussed, because I don't think it fits the concept. Ice and Rock STAB are already fantastic assets to an offensive Pokemon, so no Extreme Typing Makeover is required to make those types assets for what the Pokemon is trying to do. We would just be making Volcarona again. The actual Extreme (and I do mean Extreme here) Typing Makeover would come with trying to create a defensive Ice/Rock Pokemon, trying to turn the typing's defensive hindrances into defensive assets.
And, for the reasons stated above, this is why I think you have the exact opposite idea of what we should be doing. Ice/Rock has glaring flaws that prevent it from working on any level, offensive or defensive. However, if we can patch them up and have it work in an offensive role, then we would have made a crappy typed Pokemon successful, and have it love its great STABs, which is exactly what this concept is trying to achieve. Now, if we did want to make such a Pokemon defensive, that could fit this concept as well, but that would be taking a much more difficult, if not impossible route, that is completely unnecessary. We don't need to break the mold of theses typings to make the concept work, we just need to fix them up a bit.