is a mini ipad charger 20W suitable to charge a MacBook Air M1 2020?

Not really, since Apple recommends at least a 35W adapter, which will be slow at best.

Will 20W work? I haven't tried it, but if it does, the Mac's battery is going to charge very slowly and the adapter may get quite hot.

Ideally, you would want to use a 140W, 96W or 70W adapter with the MacBook Air

If you decide to try a 20W adapter, let us know about the results if you can.

Thanks!