No matter how you measure it, the Government of Japan is up to its neck in debt, and is still sinking.

Thing is, you need to measure the dept against some measure of economic activity ( or actually against some measure of pay back capability), and then the funny thing is that the higher the dept, the easier it is to get to sustainable levels by deficit spending. So dept goes from D to D + d and GDP goes from G to G + x d, solving for a critical multiplier x0 where the ratio stays constant,

    x0 = G/D

So the more dept you have, the less productive need your stimulus program to be to reduce dept. Additionally, for this you can replace the measures by inflation adjusted measures and you get the same result, with some dependence on the real interest rate. ( Which can be negative.)

Additionally it is surprisingly hard to get hyperinflation, just printing a few percent of GDP in new money per year will not do the trick. ( Try occupation of a major industrial region, after the end of price controls plus a lost world war.)