This is an interesting point, which never occurred to me. If* the iPad has such thermal constrictions, I’m afraid that’s another reason why we won’t see macOS running on an iPad.
(*) This is a big if, and an interesting engineering point to debate: is the M2 on the iPad Pro more thermally constrained than the M2 MacBook Air? As far as I know, the M2 iPad Pro has all the rear surface to dissipate heat.
Anyways, I think this issue could be solved tuning the M2 on the iPad to thermal throttle earlier, or simply having a slightly lower clock speed.
I don't think the thermal side of things would be too bad. The iPad really doesn't tend to heat up unless you're running things you know intuitively as a user are intensive applications anyway.
But absolutely the MacBooks are far less thermally constrained, mostly because the screen is totally separate. On the iPad all the heat generating components sit right on top of each other. There's a reason you can trigger the "will resume charging when device returns to normal temperature" easily on phones and tablets but not MacBooks.
The biggest issue probably would be battery life, with the way macOS accumulates a lot more stuff going on in the background.