Hello,
By default, the HikaShop checkout is responsive, here there is something that mess up your display in mobile.
From this, several things you can try in order to better understand your issue :
1. Try to process some test with a default template (like ProtoStar)
2. Try to switch off Bootstrap.
From Main HikaShop Configuration & Advanced tab
If use a default template solve your issue, check that you haven't some
HikaShop override view, and if not, this can mean that your root issue come from your template.
then contact your template creator support.
Note : Documentation around
HikaShop override view
Hope this will help you to progress on your issue.
Regards