remount /mnt as read-write

Use these commands

su -c 'busybox mount -o remount,rw /'
 # this will remount rootfs at / in rw mode

su -c 'mkdir /mnt/"NEW_DIR"'

 # replace NEW_DIR with the name of the directory you aim to create under /mnt

Note that in Android /mnt is not a mount point for any real or pseudo-filesystem but its sub-directories are (actually, /mnt is a sub-directory of the / (rootFS), which is why you'd have to remount that to apply changes here). Also note that mnt directory in Android is created by kernel at every boot¹ so any modification would be lost after shutdown.

_{1: mnt doesn't exist in a persistent filesystem but in a specific ramfs named rootfs - a filesystem which exists in RAM only.}