I would say that if it is enemy on enemy it would be ok. You get to a point that your damage might be too much to save the jet so taking out an enemy by raming the jet could be possible. If it is team mates knocking you off course or destroying your jet then yes it would be griefing.
Ramming an enemy aircraft if the same as jihad jeeping an enemy tank. It is taking out an enemy vehicle. Destroying friendly assets is griefing just like destroying anything else.