Except that the problem isn't the signature itself, it's the required infrastructure. Grandma doesn't know how to check The Donald's signature. And, of course, the infrastructure is hard (just check the unfixable problems with the PGP persistent DOS attacks that were discussed a year or 2 ago).

Twitter knows how to check a signature though.

I don't tweet much, but when I do I'd love to be able to sign them like I sign my git commits.

We did exactly this for a side project last week:

https://github.com/shabda/tweet-signer

At it's core it's just a spec, so feel free to write your own tools! Suggestions for improving the spec welcome. Please use Github issues