I have thought about it, and have a simple package on CRAN {types} showing how you can add types with ? https://cran.r-project.org/web/packages/types/README.html.
But the bulk of the work would be using these types of annotations to do static analysis, or doing other things like adding optional type checks to the code. https://github.com/jimhester/typeCheck was an exploration along these lines.
Aviral Goel is also doing ongoing research at Northeastern on type annotations for R, e.g. with his https://github.com/aviralg/annotatr project.
But the bottom line is none of these projects is anywhere close to mypy and it would take a considerable investment before they provide much of a productivity boost.
There have been no serious talks at RStudio I am aware of to provide a mypy equivalent, but if more community members express interest in gradual typing maybe that would change.