Will the 20w usb c charger overcharge an iPhone

No. iPhone regulates its charging. It only pulls as much power as it needs, as such it cannot be overcharged.

The 20watt charger from apple is perfectly compatible with iPhone, and for supported models will fast charge the iPhone.

Link-> Fast charge your iPhone - Apple Support

Not at all.

The phone will only draw as much power as it needs.

In fact a 20w power adaptor is ideal for ANY iPhone or iPad.