The fact that the movie its from does not, in fact, exist raises some pretty serious problems with including it in canon.
Well, we do know (from the movie that actually exists) that if it copies someone with a physical defect, like a bad heart, that it will copy the defect as well, because we see that happen. Would it do that to a blood clot? I’m not sure - the blood clot isn’t really a part of the copied creature, at that point. And I’m not sure to what extent a blood clot is still living tissue, so I don’t know if the Thing could necessarily take over the cells in a blood clot. I’d guess that, if the Thing attacked someone with a blood clot, and infected them with itself, then the blood clot would remain, because nothing its done would cause it to be removed - it just colonizes all the living cells around the blood clot. On the other hand, if the Thing monstered out, then tried to reassume the form of the blood clot guy, I’m guessing the clot would be gone, because it’s not actually a part of the organism anymore, it’s basically a foreign body.
If it took over someone with a cold, I don’t think it would copy the cold, and I’m guessing that the person taken over would be suddenly cured of their cold - it shouldn’t be able to copy viruses, for the reasons both of us pointed out, but it should be able to out-compete the virus, by just being way better at the whole, “replacing genetic material in a cell with its own DNA” thing.
That feels like a lot of extra steps versus just, “It puts the earring back in after it eats the guy.” We already know that, when impersonating someone, the Thing can wear clothes, use tools, and take actions to disguise the fact that its infected, so “It can put in an earring,” is absolutely not a stretch for it. Plus, if its growing its own jewelry, then it’s not an “exact copy” anymore, and is creating a potentially exploitable flaw in its camouflage.
Finally, if it has the ability to effectively forge trace metal in its body into specific shapes, I think you still have the “gun” problem. Because what would stop the creature from swallowing a bunch of iron ingots, and using the same process it can use on trace iron to turn it into some advanced mechanical device?