It helped it quite a bit, really. While it is not top-tier, it still is a significant improvement on the base Pokemon. If you look at Mega Beedrill and what they did, this is pretty much the best they could do from following the basic rules of Mega Evolution. Mega Evolution increases the Pokemon's BST by 100, the only exception is Mega Alakazam in Gen VI(at 90) but that was clearly an error since it as fixed in Gen VII, they probably designed Mega Alakazam's stats before they decided to give a +10 boost to its base form's Special Defense.
With such a low BST as what base Beedrill has, the main thing they did is put all of the +100 base stats into Attack and Speed. Furthermore, while some Megas get a speed drop to put more into other stats, they instead did this with its Special Attack, and rather than the -10 penalty of most of the speed drops, they did -30 to it and put that in for more Attack and Speed. Finally, since its offensive boosts still wasn't enough, they gave it Adaptability to boost is STAB moves further.
Outside of giving it Huge/Pure Power as an ability or draining its Defense and Special Defense for more Attack and Speed, this is really the best Beedrill could ask for with Game Freak's self-imposed limitations. And even putting it alongside most other megas, it was heavily Min-maxed. All of the XY Megas and most of the ORAS Megas had spread-out or sometimes useless stat buffs, but they didn't do anything like that for Mega Beedrill. The only other mega to do that is Mega Slowbro, who only got Special Attack and Defense buffs.