Well, it depends on your strategy.
Mega Houndoom still gets the standard base stat increase given by Mega Evolving, but its ability becomes useless without harsh sunlight. Because of this, you're better off with a different Mega, or you could teach Houndoom Sunny Day (uses up a turn and a moveslot, but may be worth it).
So no, Mega Houndoom isn't worth it without bright sunlight since its ability is based upon it, and there are many other Mega Evolution choices you could make.
