It’s an okay idea. If you have a React and React Native project this can be a good strategy to share code. I haven’t found there to be much code sharing needed between the client and server in practice. The auto type generation does most of the sharing needed. Maybe for things like form validation it could be helpful if you use a package like Yup on client and server side.
I wouldn’t do it as a default just because any time I’ve worked with Lerna in the past it added unnecessary complexity, and others I’ve spoken with have had similar experiences. I’d definitely look at using it if your project will benefit from it though (as with React/RN).