I am now trying to wrap my mind about those rules - Thorn Whip is a Melee Attack? - 5ed is hard!
Well, it's a melee spell (it might be best not to think of it as a melee attack which I think has a technical meaning in 5e when it comes to things like fighters' Extra Attack). That is, it's one that doesn't have any targetting disadvantage when cast within melee distance of an enemy, unlike, say Hex or Fire Bolt or shooting them with an arrow. For me that's it's main selling point as a cantrip. It has a bit of range but the flexibility to cast up close if needed. Of course, if an enemy is in their face many druids will want to get their Shillelaghs out instead, but still.
As an aside, it's not clear from its tooltip that Thorn Whip is considered a melee spell for the purposes of determining disadvantage at close quarters, unlike, say that for Shocking Grasp which clearly says "Melee", as the tooltip only gives the range. I personally only figured this out from guessing and testing it (you can see when you hover over a target for a spell any sources of advantage or disadvantage including "Target is too close"), but feel like the game should make it clear. I'm not sure whether it's the only spell that has this ambiguity being a melee spell with a bit of range, but it's the only one I've noticed so far).
