You should be paid for everything you do in the store. That includes chalkboard art.

You should absolutely be getting paid. I do signs at HOME for the store and my manager insists on paying me for my time.

Similar to coaching conversations, they’re only to be had when you’re clocked in, so yes, you should be paid even when you’re doing chalkboard art

Doing chalkboard art for the store is work and you should be clocked in to do it. You shouldn't be doing it in unscheduled time without approval from your SM.

Whenever it gets really slow I often ask a barista to do some chalk art.

You’re doing something for the store you wouldn’t normally do if you weren’t there, so yes, you should absolutely be getting paid.