When bathed in the energy of Mega Evolution, Shellder converts into impregnable armor. There is virtually no change in Slowpoke.
– Pokémon Moon Pokédex
According to multiple Pokedex entries, the only part that changes in Slowbro when it mega evolves is the Shellder part; the Slowpoke part is not changed. Also, apparently the non-Shellder part of Slowbro is actually just Slowpoke, hmmm…
I find it a bit strange that we only got Mega Slowbro, but not Mega Slowking, even though we got both Mega Gardevoir and Gallade, and both Mega Latias and Latios. Maybe ’cause their stats would be too similar?
Stat-wise, all the stat gains Mega Slowbro gets all go into Defense and Sp Attack, giving it whopping 180 base defense. This, along with its ability Shell Armor which guards against crits, makes it really bulky.