It’s a valid idea for posts with original content. With all the pearls I cast before swine, I probably should at least put a CC-BY-ND license on my—ahem—humble screeds.
If you’re suggesting this becomes a part of the UI, unless you know how to program I’d say get in line.
I don’t know (because I don’t really care) if over in the Lemmy laboratory they’re working adding an email-like automatic signature option but if so, you could always add your license of choice as a signature. Ciao!