If landowners have to pay ground rent then they will sell any land that is not making money. This means the market will be flooded with new land. So the price of land will go down. So mortgages on land will go down.
Ground rent means work is not taxed, so it's cheaper to hire builders. So building becomes cheaper, and mortgages on buildings go down.
Ground rent means nobody speculates on land, as there is no unearned profit to be had. So after the initial price drop, when ground rent settles in, prices will grow at a steadier pace. So mortgage deposits will be smaller (as they exist to ensure the lender against price volatility).