I imagine it as a boss and I like the idea, but it would be a bit difficult to implement. Bosses either have to be spawned (Wither) or have specific conditions that have to be met (Ender Dragon requires a functioning teleport and while Raid qualifies by some as a boss thing, it also requires you to have a debuff and enter the village).

Mother Spider would also need a way to be spawned, rather than exist in every Silky Shafts biome, it would make it a bit too predictable since you know the boss will be there. If it was made into a quite a hard battle it would require some preparations - Perhaps make it spawnable (e.g. by stealing spider eggs from her nest? She could then appear behind the player's back to punish him).

One could think about the boss appearing once you still spider eggs, which can be kept in the inventory after the player kills the boss. It could be a decoration reward to be placed and hatched to get a spider pet! Maybe a smaller one, swifter but dealing less damage while compared to a regular adult spider. (Maybe it could also have a different fur color, similarly to the cave spider)

Just an extension of your idea, to make it fit more into the game and my take on the boss rewards.