A fall in treasury yield (Good or Bad).

Treasury yield according to me, is return on investment in treasury bills. So, a fall in such a yield is definitely bad. As the trust of investors in the government of that particular country would get hurt.

Now lets look at the sources for Inflow of cash for the treasury of any country 

1. Taxes

2. Treasury bills

So, I think a low in such a yield would imply that the tax system of that country is not functioning well.