Apple - Can I (or should I) use a MacBook charger on my iPhone?

Unless there is some reason to the contrary that I'm not aware of, yes, you can use your MacBook charger with your iPhone. The power adapter will only convert as much power as is drawn, and the iPhone will only draw as much power as it needs.

(Note that it is not a good idea to charge your MacBook with an iPhone or iPad charger!)

This video shows that you can use MacBook 12" 29W USB-C Power Adapter with iPad and iPhone. https://www.youtube.com/watch?v=fWzbaH24Q0Y