Mantle Do I Need This?

Yes - if a mod requires it. Mantle is a hard dependency for Tinkers' Construct and other mods that build on it. Those mods will not load without Mantle present.

No - on its own. Mantle by itself adds nothing you can see or use in-game. There are no Mantle items, blocks, or recipes to find.

If your modpack or a mod's requirements list Mantle, you need it. Otherwise you do not.