At least it would be reasonable to show a message like "Sorry, no mobile support yet".

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.