Terraria – host a Terraria server on the iPad then play it on a PC

It is certainly possible. You both just need to have the same mods active.

However, you say you're using the Obsidian mod, and from what I remember there is a part of that mod that is strictly single-player only (specifically, the Parallel Worlds pack), so it may not work as you want it to. Otherwise, running mods in multiplayer is fine.

The mobile and PC versions of Terraria have different content and are not compatible with each other. This might change in the future, but it seems unlikely at the time.