Houses appreciate in value, starships depreciate... the mortgage is a silly method to finance an asset that loses value.
If they depreciate in the short term possibly. If we are to believe the CRB depreciation is very low in Traveller as you can sell a ship after 5 years for what you bought it for. The next 10 years it depreciates at 1% per year and then 0.5% each year for the next decade and it drops off even more after that.
The lender only cares that your ship is worth more than the amount you have yet to pay off. That way they can seize the ship to cover their capital.
Traveller ship mortgages run a shade under 4% APR. For a MCr100 ship:
In the first year you only pay off MCr1.039 off the capital. Your ship is still however worth the full MCr100.
At the end of year 5 you will still owe MCr94.365 but at the start of year 6 your ship is still worth MCr95.
By the end of Year 10 you owe MCr87.492 and your ship is worth MCr90.
At the end of year 15 you owe MCr79.108 and your ship is still worth MCr85.
In Year 25 you owe MCr56.408 but your ship is worth MCr80.
At the 25 year point you will have shelled out MCr125 and the lender will have made over MCr69 off you.
When you finally pay off the mortgage in year 40 you will still be able to sell the ship for 80% of what you paid for it.
You are not buying the ship for primarily capital investment, you are buying it as a tool to generate income (via trade or piracy), but even so you can make a tiny profit even selling at the top of the market. You just have to meet your running costs, but not the mortgage to turn an overall profit as you get that back plus a premium if you sell the ship at any point. The premium grows the longer you have held the ship (and it is best to sell it just before the step drop in value (at the 6, 11, 16, and 26 year points).
After 1 year you make MCr1.039 (which is very poor but still better than the return on a ship share).
If you sell it at the 5 year point you make MCr5.635.
If you sell it at at the 24 year point you make MCr25.895.
Clearly if your ship can clear both running costs and mortgage then you can run without a significant cash reserve.
Loans secured on depreciating assets are common in business in real life (or Hire purchase / rent to own arrangements) and make sense as long as the asset helps you make more money. I think we used to work on 20% depreciation each year for tax purposes so Traveller depreciation is very low.