"Terrorist" is a label that is given due to the nature of the actions, not due to the goals of the organization. In the Syrian war, al-Nusra and Daesh also sometimes battled each other, but none of them ceased to be a terrorist organization.
If you found an organization that subscribes to a world of love, peaceful coexistence, and human rights, and try to achieve this by terror attacks, you have founded a terror organization. This is independent of the goals, and thus it is irrelevant to the question if Hezbollah was effective (or more effective than the US army) against Daesh or al-Nusra.
A terror attack is by definition an attack whose primary goal is to instill terror and fear. Hezbollah has repeatedly carried out such attacks in the past, and thus is listed as a terror organisation by the EU.
(Of course, in reality politics is much more reluctant to act if the organization is more in line with the own interests than if it poses a threat; compare the very different actions in the EU against Daesh with those against the Kurdish Worker's Party (PKK), although both are listed as terror organizations.)