I think the Mezzo's stem design is quite clever. Rather than go up, then out, it simply goes out, then up. This enables a tight fold without nearly the compromises that Brompton needs to make.

I think design aesthetics are often bunk. Perhaps disagreement with the aesthetics of the stem might stem from spending one's life seeing stems go up, then out; or just up. If the world only had Mezzos, surely (say) a Brompton or Tikit would look ridiculous.

This is the same effect that recumbents have on people. Recumbents are in many respects superior bikes; certainly much faster, more comfortable, and safer with forward lookahead. But they look ridiculous because people are so used to seeing, for 100 years, a largely inferior design, and it's burned into our neurons as the "better designed bike".