Note: this is probably the shortest post I've ever written
According to Larry Summers, secular stagnation is supposed to be reflected in the continuous decrease of bond yields since the early 1980's (the increase before can be explained in a simple way-inflation):
Now remember where bank capital and most of the funds of the pension funds industry are (most times forced by legislation) invested: Bonds. I'd also like to remind some of the supply and demand premise in economics: when supply of funds increases then price increases and thus the yield decreases. And we are on a continuous search for safe assets to invest in. As the safest of all are government bonds (explained here) the increase in life expectancy has forced pension funds to invest more and more assets in the bond market (for a more detailed view of pension systems and population read this).
Concluding, I do not believe that stagnation can be inferred from bond yields. Declining returns just mean that the supply of funds for bonds has been stronger than demand and this is what's driving yields down. Now if that can affect the economy is a different story (one which I do not really trust to be honest since the US has been growing steadily since the 1980's), yet it does not indicate stagnation on its own. It just shows that when times are bad, people come up with a lot of explanations about what's at fault. But then again, I might be wrong and Summers could be right; time will tell.