Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Performing funeral prayer at graveyards, when there is a need for that, is permissible.
2- As for other prayers, graveyards are not meant for that, and this should be avoided, generally speaking.
Answering your question, Sheikh Mustafa Umar, President of California Islamic University, states:
Performing funeral prayer at graveyard, when there is a need for that, is permissible. This would happen when the funeral prayer was not performed in a mosque or elsewhere and the body reached the burial site already.
As for praying other prayers at a graveyard, this generally should not be done since the Prophet said, “All the earth is a mosque apart from the graveyards and bathrooms.” (At-Tirmidhi and Ibn Majah)
Almighty Allah knows best.