As stated in the description for the thorn whip cantrip (PHB, p. 282), 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.
The third benefit of the Mobile feat says (PHB, p. 168):
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.
Given that the third benefit of the Mobile feat only requires a melee attack to be made to avoid provoking opportunity attacks (without specifying the type of melee attack), would that cantrip fulfill the trigger for Mobile?
In other words:
Is a spell melee attack considered a "melee attack" for the purpose of the Mobile feat?