Ok, I've made quick tests (I've got guests home today ). I've tested four different USB PSUs, all rated 5V >=2A.
With two of them, I get 48°C idle. With the next two others, I get 51°C, so I'd say that although there may be some effect due to the PSU itself, there isn't any significant variation.
After that, I've retried the first PSU (the one that got me 60°C results), and now I get constant 49°C! What I'm wondering is that if somehow, the higher temperature wasn't caused by the first run of the Debian? Maybe some sort of background process was left running that caused it? I have three other OPi Zeros, so I'll definitely test things further. I admit this left me with some head-scratching