The Devil's Tongue trait simply lists Vicious Mockery as being known, so I'd assumed the damage would increase as per usual (5th level [2d4], 11th level [3d4], and 17th level [4d4]), but it seemed important to get a definite answer on this so I don't make a mistake at the table.
Short answer: Cantrip's damage is affected by character level, no matter their origin.
Not-too-long answer: Cantrips increase damage as you gain character levels, as the description of Vicious Mockery explains. Since Devil's Tongue doesn't give any extra rulings in this regard, it's safe to assume that the damage increases as you level. In fact, the same is true if you get your cantrip from a class and then multiclass, the damage is calculated per character level and not class level, as answered here.