Please Don't Excuse My Typos
It amazes me how I still get emails with the signature: "Please excuse any errors as I typed this on my phone." If that is passable, so would the signature: "Please excuse my poor grammar, I was in a hurry."
A smaller screen does not impair a quick review of your email. Read over what you write. Make sure it's concise and there are no spelling errors. Anything that makes your message harder to understand should be changed or removed.
Take pride in your professional communications. Write well and effectively. Don't let your grammar and spelling change with the device you're using.