Nitpick time: File this under “wrong usage of -eth when trying to sound medieval”. That particular usage became “-es” in modern English, and if you make that replacement in this comic (cometh → comes), it’s immediately clear that it’s wrong. “Come onward” would have been just fine, but that, of course, looks far too modern.
I mean, you could read it as being deliberately demeaning or objectifying - she is being a hard taskmaster - but I don’t think that was the intention here.
If she has permission - or dares take the initiative - to use the familiar form of address, she could try “Now, come thee onward!”, keeping both that “th” that was wrong before, as well as the syllable count. Might still be a bit weird in context, but not grammatically.
She does say ‘thee’ in the other panel so familiarity is established.
Anyway, even without knowing the rule, you can sense something is wrong by saying it out loud. I don’t know if there are examples of turns of phrase staying in use when they don’t flow well in spoken language?