I can't answer your mortgage question, except that most lenders want your housing expense to be less than 35% of your income.
As to the national debt, yes, the debt to GDP ratio is often given as a way to say that the US debt is not that bad. But the fact is that this ratio is at it's highest point since WWII. GDP is at $18 trillion, debt is at $19 trillion.
One thing for sure, running a deficit every year, adding to the debt every year is not good fiscal policy. Which is why I used the analogy of digging a hole, hitting something solid (the 100% debt to GDP ratio, for instance) and then just keep on digging is wrong.