You can always have a look at the API documentation txt file
https://wiki.esoui.com/APIVersion#live_API_version
-> link to "API TXT Documentation" .txt file -> Download and open with a text editor
and search for the function name "GetTargetMountedStateInfo" in it.
It will show you the parameters to use and the return variables then:
Code:
* GetTargetMountedStateInfo(*string* _characterOrDisplayName_)
** _Returns:_ *[MountedState|#MountedState]* _mountedState_, *bool* _isRidingGroupMount_, *bool* _hasFreePassengerSlot_
So this function returns 3 parameters.
If you do not need the 1st one "MountedState" you can use the placeholder _ to show that, correct.
And the other 2 return values are _isRidingGroupMount_ and _hasFreePassengerSlot_.
Your code looks okay to me.