This mega doesn't have a lot of versatility in its optimal set, but it truly excels as a sun sweeper/wall-breaker. Under sun it does an absolutely INSANE amount of damage with eruption, and 125 Speed isn't anything to scoff at either. Though it may not be the best Mega Evolution objectively, it gets above average coverage and a hell of a lot of damage.
Here's a couple of calculations just to show the raw power of this Mega Evolution.
252+ SpA Mega Typhlosion Eruption (150 BP) vs. 252 HP / 4 SpD Eviolite Chansey in Sun: 301-355 (42.7 - 50.4%) -- 1.2% chance to 2HKO
252+ SpA Mega Typhlosion Solar Beam vs. 252 HP / 0 SpD Mega Milotic: 216-256 (54.8 - 64.9%) -- guaranteed 2HKO after Leftovers recovery
All in all, Mega Typhlosion is a decent sweeper/wall-breaker that thrives under sun, but has a lot of weaknesses and needs a lot of support to clean up teams with Hubris. That being said, it's a volatile threat that can just ruin teams when left unchecked by blowing up and uncontrollably erupting on your opponent's Pokémon, which makes it awesome in terms of flavour.