Yes, of course a shooting that leaves five people dead in a Mexican resort is an act of terrorism. It's not an accident. It's not justice. It's nothing but violence, and violence is terrorism. It doesn't matter who is behind it. An intentional shooting is an act of terror. Its purpose is to create fear and hurt others.
The very word terrorism is very misunderstood and many think that it is only linked to large events where a large number of people die or in cases where war or religion is involved. This is not the case and as in this case in Mexico it relates also to several people dying in an event where someone intended to cause mass murder.
No, not every shooting is an act of terrorism. There is too much talk of terrorism every time there is a shooting. The solution for shooting is gun control not combating terrorism. Most shootings such as this one can be pulled off by one person with absolutely no terrorist ties.
Right now, it's being reported that there was a lone shooter and that authorities don't believe it to be an act of terrorism. It is a horrible and devastating act of violence and it is tragic, but without more information, it doesn't appear to be an organized or unorganized terrorist act.