Hi,
That specific popup is using "Squeezebox" which is a mootools popup system provided by Joomla itself.
We do not have a lot of "control" on that popup and how it is reacting on small screens.
But one goal of the next major HikaShop release is to not use that popup system anymore.
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.