It depends on what's driving it.

Many families in the developing world are having fewer children because infant mortality is declining and it doesn't take as many pregnancies to reach their desired family size. That seems good.

In the developed world fertility rates are declining in part because women have more options for what to do with their lives, which also seems good, but they are also declining because the family is being severely devalued.

That last point is the really bad one. Having recently started a family (and being someone who might not have), I find it really tragic that people are missing out on this part of life who might have wanted it.