Is AppleCare Worth for iPhone 12 Mini or my 2017 MacBook Air?

See this Apple article about AppleCare to see when you can purchase AppleCare. How long after the purchase date varies depending on the device and where you live.

As to whether it is worth it: I have always purchased AppleCare for my notebooks but usually not for desktops. Notebooks are extremely expensive to repair and design restraints make them more likely to fail. I generally purchase AC for a desktop only when it is a brand new design; I consider them a bit more risky. Over the past 20+ years I’ve gotten my money’s worth from AC, just barely, but I consider it decent insurance. For phones and iPads I didn’t purchase AC for a long time. But in recent years Apple has allow AC to cover damage (to a limited degree) which has made it more attractive.

FWIW, you cannot get AppleCare for a 2017 Mac.

It is only available for new Macs and depending on where you live

anywhere from 30 days up to 1 year after purchase.

Well, it's insurance - same as if you were to buy homeowner's insurance: hopefully you will never need it, but if there is a problem, it'll save you lots of money.