It is Makrooh to trim or cut the unwanted hair or nails in the state of
impurity. (Bahishti Zewar vol.2 pg.83 ZamZam)
This would refer to the period of Nifaas as well. However, it is also
Makrooh to allow the unwanted hair and nails to grow for more than 40 days
or so long that water does not flow freely over the skin under the nails and
that dirt does not accumulate under it.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai