I dont think number of full nodes signify anything about adoption. There are 122 Chinese full nodes as opposed to 290 of UK. But, Chinese volume (which can be treated as a parameter of adoption) is far far higher than that of UK.
I think it's a reasonable metric. I assume you can't mine without a full node and you'd be a bit insane to run an exchange without one.
Neither the UK or China have much real adoption. China has zero utility other than exchange gambling and mining. The UK has hardly any businesses accepting and no proper exchange.
So let's look at a country which does have a hint of real adoption such as the Netherlands. There are 329 nodes there.