This also looks like an opportunity to improve ZeroSSL and any other clients to help users more - if the user typed something that doesn’t seem like a domain name, ask them to make sure it’s really a domain name and not something else. e.g. if it has a colon in it, that can’t be a domain name, but it could be a URL, if it has an @-symbol that can’t be a domain name either but it could be an email address.
A really good way to improve the UX of software is to study situations where a real user doesn’t get what they wanted, and ask what your software could do to help them.
It makes sense to do this work in the clients (even though there are lots of those) not the ACME server because the ACME server is security critical, it should be as small and simple as possible while doing the job correctly, whereas clients are not so security critical, so if they want to have a whole sub-routine that spots URLs and says “Hey, did you mean this domain name instead?” that can’t worsen overall Let’s Encrypt security.