As a banner or something, maybe. But I'd much rather see a broken-looking site on mobile than see nothing at all on mobile.
In the same category: websites that display nothing but a splash screen "This site requires Internet Explorer X" or whatever. Don't nanny me, just feed my browser the HTML! Whether my browser can render it properly is my problem, not yours.
the overall reason why we didn't ship a mobile site is that code is inherently very hard to read on mobile
and i think to do that properly, you have to design explicitly for it. (and that is not, in fact, something that i do want to vibe code)