That's super, *super* situational, though, which effectively only works on a handful of Pokemon with Trace. It can also potentially backfire in an infinitely more situational case: Say there's a Gardevoir that traces Slow Start, but it uses Skill Swap to send Slow Start to your Mewtwo, and then it Mega Evolves next turn. I don't think a scenario like that would ever unironically happen, and there are arguably better ways to mess up opponents with a bad ability. (Durant gets access to Entrainment to pass Truant to opponents)
So yes, Slow Start can be used, but being viable? I'm a little more iffy on that. You're better off with other means of speed control like Tailwind and Me First to directly help your entire team, instead of running a dead slot to take on only three potential species of Pokemon, those being Mega Alakazam, Porygon2, and Gardevoir, assuming they run Trace for the latter two.