While it's true that there are better ways to stalk people, I think an AirTag brings something to the table that wasn't there before: the normalization of having stalking hardware at hand.
Buying a "stalker kit" requires me to think long and hard about stalking someone. I need to make the decision, buy the hardware, wait for it to be delivered (or, alternatively, walk into the kind of store that sells spy equipment), create an opportunity, and go for it. Most well-adjusted people wouldn't go so far.
But once AirTags normalize having these things in your keychain all the time, the "entry barrier" for stalking is lowered. All it takes is one beer too much and a split-second decision.