Despite its great offensive stats and Ability, Mega Beedrill is most crippled by its typing, Bug/Poison, which is far more horrendous than you may think. These types fail to break through common defensive Pokémon in the meta, such as Hippowdon, Buzzwole, and virtually any Steel-type. None of its STAB or coverage options help against Pokémon common to the meta, such as Skarmory, Celesteela, and Scizor, who all wall it. These Pokémon often force Beedrill to pivot with U-Turn, which, as a Bug-type that can't hold Heavy-Duty Boots, leaves it crippled against Stealth Rock more often than not. Therefore, Beedrill needs a lot of hazard support to both finish off their opponents and allow it to safely pivot in and out of the field, which means you've basically got to build your whole team around the bee.
Lokix far outclasses Beedrill in terms of everything, so it's best to use it instead. Not only is it not confined to using a mega stone, allowing it to run Heavy-Duty Boots as a U-Turn user and allow the use of another mega on your team, but its got better STAB types in the form of Bug/Dark. Additionally, it's actually faster as a result of access to powerful priority moves that are rendered unresisted thanks to Tinted Lens, in the forms of First Impression and Sucker Punch. Lokix is the Pokémon Mega Beedrill wishes it could be and, not relying on building your whole team around it, is a far better option on an actual team.
The National Dex UU Viability Rankings on Smogon lists Mega Beedrill as D-Rank, while Lokix is B-Rank.