«إِنَّمَا نَحْنُ مُسْتَهْزِئُونَ»
We are only mockers.
A nominal structure with an active participle can foreground a state or established characterization. By contrast an imperfect verb may highlight the occurrence or recurrence of the action.
The issue is not word substitution alone: performing an act and presenting oneself as characterized by an act are distinct rhetorical modes. The discussion of changing forms after law uses precisely such contrasts.