Is there any API by which can create a new user in Sitecore with JSS?
It does not exist now. For that you would need an web API which also contains some backend validation to not blow up your user database. In backend you can easily create users using Sitecore API.
At some point maybe they will have such a function in JSS. I can imagine this function as part of Sitecore Forms support by JSS in the future.