"Buy me a coffee" as in-app purchase?
Yes. This kind of 'donation' IAP is fine and approved by Apple.
This has always been possible, but Apple also clarified this with an update to the App Store Guidelines in February 2021:

3.1.1: Clarified how apps can enable customers to “tip” developers: “Apps may use in-app purchase currencies to enable customers to “tip” the developer or digital content providers in the app.”

For an example of an app which uses this, see Pedometer++ (an app promoted by Apple on numerous occasions) which uses three donation IAPs of different values to allow users more control over the quantity of their donation—also allowed.