Is there a good way to generate JSONschema and thus forms from schema.org RDFS classes and (nested, repeatable) properties?
Depends on the purpose here. With this, my goal was to speed up the Write-Update-Debug development loop. Depends on the use case, simply using Google's Structured Data Testing Tool  might be a better way to verify schema than JSON-schema?
It should be possible to model the basics (nested structure, repeated structure, defining the "domain" and "range" of a value).
Schema.org definitions however have no conception of "required" values, however, so some of the cool form validation we see in some of these tools might not apply here.
[*] _Consumers_ of Schema.org JSON-LD or microdata, however, might define their own data requirements. E.g. Google has some concept of required fields, which you can see when using the Structured Data Testing Tool.