Yes, I think it was an act of terror. In most cases, shootings don't occur for any other reasons. If someone is willing to go and shoot a German theater, it is almost certainly an act of terror. Of all shootings to occur in recent history, when hasn't it been?
A movie theater in Germany was recently the site for a deadly shooting. I believe that this was an act of terror because events like this do cause terror: they create fear in the hearts of those that survived. Not only that, with ISIS becoming more prevalent, it is very likely that that terrorist organization was involved.
The German theater shooting could be considered an act of terror. Terrorism does not have to come from a specific terrorist group, but can be taken out by a lone wolf attacker. The purpose of a mass shooting, or an attack at a public place like the German theater is to create mass hysteria, terrorizing people.
Shootings in Germany are rare, but acts of terror are done by a group of people who are looking to incite fear and panic. In most cases, terrorists or their group will step forward and claim the act within a matter of hours. No one has come forward, so it is likely that this is simply an isolated, yet tragic incident.