In German this is a usually pejorative term to describe activities somebody (often people in a position of power but accountable to the public, more rarely subordinates under pressure from above) undertakes to be visibly seen as "doing something" to address a problem, but the actions are not well thought out, unlikely to really solve or improve the situation, but are (at least from the perspective of the speaker making the charge of something being "Aktionismus") done wholly or partly to provide cover against the accusation of inactivity or indifference.
Like when you hastily implement "security theater" measures against a real security threat, because you can't think of any actual solution to improve security for real, but not doing anything and admitting to having no solution would be politically very costly. Opponents then might accuse you of "Aktionismus".
So "Aktionismus" is a general term for hasty, thoughtless responses of this type, used by opponents of the actions. It is often coupled with "blind" as in "blinder Aktionismus" to emphasize the lack of plan or vision to arrive at a real solution.
It is similar to the accusation that something is merely a "symbolic action" but that implies more a deliberate gesture lacking concrete results, whereas "Aktionismus" is more of a harmful flailing around.
"Aktionismus" has also some sort of specialized meaning for some performance art movement from Vienna, and apparently the word "actionism" exists in English in the translation for that Austrian art, but it doesn't seem to be used in the colloquial sense. But clearly this is a common phenomenon (and accusation) in politics, so there ought to be an English term.