What happens when Merfolk Trickster's ability targets Tempest Djinn
Yes, it's a so-called 'static ability':


  604.1. Static abilities do something all the time rather than being activated or triggered. They are written as statements, and they’re simply true.


The ruling under the card in Gatherer confirms that:


  Tempest Djinn’s ability that modifies its power applies only while it’s on the battlefield. In all other zones, it’s a 0/4 creature card.


(emphasis mine)

so it's affected by Merfolk Trickster.