What is the ruling on wearing hijab in Ramadan only?

Hijab is obligatory in Islamic law; it is obligatory for Muslim women to wear it both during the blessed month of Ramadan as well as at other times.

Repentance and Ramadan
Ramadan is the month when Muslims are expected to increase their tawbah [repentance] and turn over a new leaf, making it the starting point for doing righteous deeds that will guide them to God the Almighty and ultimately gain His pleasure.

The ruling
A Muslim who is granted success in worshiping God the Almighty and obeying His commands in the month of Ramadan, must continue in the same path after Ramadan. This is because success in observing good deeds is a sign of having them accepted.
And God the Almighty knows best.
 As for the juristic issue of the validity of the fast without wearing the hijab, the fast is valid if all its integrals are fulfilled. Wearing a hijab is a separate act of worship and not wearing it does not have a negative effect on the acceptance of fasting.
Allah the Almighty knows best