That would be acceptable
If a Vice-Admiral beats Mihawk in a legitimate 1v1, that Vice Admiral will be admiral/Yonko level.
If Shiryuu beats Mihawk in a legitimate 1v1, Shiryuu will be admiral/yonko level.
Mihawk is already 100% confirmed on par with the Yonko & Admirals. Any denial is simply going against the confirmation of him being somebody who stands at the top of the OP world along with the Admirals & Yonko. At this point this just serves as a cute discussion where fans can go back and forth in.
What wouldn't be acceptable to say is:
- that Mihawk isn't Yonko level because he lost to Shiryuu, when you had no idea how strong Shiryuu was.
- that Mihawk isn't admiral level because he lost to a Vice Admiral, when you had no idea how strong the Vice-Admiral was (now if it's a vice-admiral we've seen proven to be a scrub like Maynard and such, sure Mihawk won't be admiral level.. but he'd never lose to somebody that isn't admiral or yonko level).