JSONAPI is part of the wider family of open standards: http, json, mime, etc. I think using a separate mime type for CSV is a perfectly valid solution, not in conflict with JSONAPI at all.
By analogy: you usually wouldn't use JSONAPI to serve images. You might use JSONAPI to get URLs that point at images, but then you'd use HTTP with an image-appropriate mime type to load the images. CSV can be handled the same way.