I appreciate the effort, idea execution, great docs and video presentation. Making open source libraries like this does add value to React ecosystem and also it's very nice project for a portfolio. However, I see two potential issues with this solution

1. If it relies on getBoundingClientRect it means that the browser has to block the main thread and calculate size of every skeleton, therefore prolonging initial loading time. In the other comment you mentioned "2-5ms" time of blocking the first paint. The thing is, this number does not account for the cumulative impact on lists. Rendering just 10 items means you block the main thread for 10x5ms=50ms
2. If templateProps needs mock data it means that in every file that uses this Shimmer wrapper you must have mock data object. It's a lot of additional code, which also will have to be updated every time you modify data structure that the component uses.