As stated in the description for Thorn Whip, the caster makes a "melee spell attack" against the target.
You create a long, vine-like whip covered in thorns that lashes out at your command toward a creature in range. Make a melee spell attack against the target. (PHB, pg 282)
Given that the Mobile Feat only requires a melee attack to be made to avoid provoking attacks of opportunity, without specifying the type of melee, would that cantrip fulfill the trigger for mobile?
When you make a melee attack against a creature, you don’t provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not. (PHB, pg 168)
Short version: Is a spell melee attack capable of being treated as a plain melee attack?