Hi,
In fact, the override address is the same than a classical one :
templates/YOUR_TEMPLATE/html/com_hikamarket/mangopaymarket/
The MangoPay plugin is using the "plugin controller" interface which allow to move interface to a plugin ; but for the point of view of Joomla it is a classical view so the override is the same than if the view wasn't in a plugin.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.