Is typst a good tool for something like a flyer (eg, printable respecting fold lines) or more generically one-page posters?
I see PDF as a blessed output, but it seems mostly in context of longer form typesetting-heavy workflows (books, papers), rather than design-heavy.
Probably not. As you point out, it's a typesetting tool like LaTeX, not a page layout tool.
You could probably write a template that does the right thing, but that's true of LaTeX as well.
Depends on what the poster is. If it's still mostly text content, just arranged in a certain way, then by all means you can. For example:
https://typst.app/universe/package/poster-syndrome
And here's one for foldable cards:
https://typst.app/universe/package/keepsake
(Not endorsing those specific packages, they're just the first ones I managed to find)
For more free-form layouts, I just use Inkscape. It has built-in PDF export which works great. I assume other similar tools like Figma or Illustrator would also work well.