Mega Heracross is bad from just a thematic view.
He's slower, which is okay, because he's more bulky and stronger.
But therein lies the problem. He's not only weaker than Guts Heracross (185 attack at max is less than 125 guts boosted, and the latter even protects you from status like sleep or paralysis)
But he's also still keeping his 4x weakness to flying, in this metagame, and still has a 2x weakness to psychic and fire. Which would be fine with the higher defenses, but he only gets 10 extra special defense. That's not enough to save him from an air slash, a psychic, or a fire blast.
So he's weaker, he has just as hard of a time dealing with everything he had a hard time dealing with before mega evolving, and then he's still slower.
But then the kicker is that he's more defensive, but loses his only form of reliable recovery in leftovers. Regular heracross can run substitute+leftovers and have like 5-6 subs even if facing something super effective
Megacross can't boast that at all. The only defensive set it can run is bulk up+rest, and that's two moves with the latter pausing you for two turns. If you run sleep talk, you have a single attack.
They just basically had no idea what they were doing while setting up its stats.
