Something can be both a terrorist attack and a hate crime.
This arson attack on a mosque is a hate crime because it appears to have been motivated by hatred for Muslims.
It would also be a terrorist attack if its intent was also to cause wider fear and alarm among Muslims, not just those belonging to that particular mosque.
Hate crimes are investigated by local police, terrorist acts by the Counter Terrorism police.
I guess if this arsonist belongs to a wider network of people planning and carrying out attacks on mosques in order to intimidate Muslims it could be upgraded. If the attack had been more serious against people (like the Finsbury Park Mosque attack in 2017) then it would also have been upgraded.