I think this is very dependent on context. For example, if we both know this is a one-off interaction and we're unlikely to see each other again, then it's good

I think this is very dependent on context. For example, if we both know this is a one-off interaction and we're unlikely to see each other again, then it's good manners to give a shallow illusion of attentiveness, and use just enough mental energy to handle the interaction appropriately. On the other hand, if we have an ongoing connection, then I think it's pretty shitty to be dishonest about how much attention you're giving the conversation. It's even OK with me to switch from the first strategy to the second mid-conversation if the situation changes, with something like "Sorry, I was on autopilot for a minute. I think I'm following now."