This question is slightly broad. You can almost think of it like “do CEO’s create long term value?” some do and some definitely don’t. It depends on many things. Generally speaking most of the actions activist investors push for are to create value and with the exception of some actions, it is to create long term value. Pushing for the below are generally good for long term value:
Changing bad management/board
If one is focusing on an event-driven investment strategy (waiting for the activist's actions to be executed and then exiting once the stock price has appreciated), then the question of long term value creation for the purposes of investment strategy don't apply.