When you create a filesystem that supports file ownership, its root directory starts owned by root (with all the `mkfs` that I remember seeing). The ownership of the mount point and the user who did the mounting are irrelevant to the ownership of the root directory (or any other file) on that filesystem. It would be problematic after all if mounting a filesystem in a different place changed the privileges required to access each file on it.
If you want to create files as a non-root user, you'll have to give that user write permission to some directory on that filesystem.