I already use a gamepad with Terraria... albeit with limited success.
I use the Steam Controller. As it emulates both an Xbox-style pad and a mouse/keyboard, it technically can* work out-of-the-box with most games.
*technically can, because sometimes games for keyboards use the whole thing, or a fair chunk of the buttons, and setting something like that up for a gamepad of any kind would take a minor miracle, so those games oughtta be ruled out. There are other games where it's entirely possible to set up the Steam Controller for them, but... there's no imaginable situation where using it would be a good thing.
I love the damn thing, so I'm hardly going to be giving it a hard ride, here, but even in its place as, in my opinion, the best possible gamepad you can currently get, Steam Controller isn't going to do well here. I feel like Terraria is one of the games out there that you'd have to tear inside out and completely reimagine before it becomes a worthwhile thing to try and control it with anything but kb/m.
I own the PC version, 360 version and mobile version. PC version is the only one I can bear playing for more than 10 minutes at a time. The 360 and mobile interfaces are changed to better accommodate the control schemes, but I don't think it helps very much in terms of ease of play and enjoyability. I can't see the PC version improving on that very much with controller support, and I doubt they'll even go as far as changing the interface to make it that slight bit easier to navigate.
Don't get me wrong, I'm not opposed to adding controller support. More features can only be better, right? I just don't see it benefiting many people, if any people.
EDIT: To clarify, I use the Steam Controller for most of my gaming, but not for Terraria. I tried it out, for a while, but it was painful. The flexibility and customisability of the controller means it could be just as easy as kb/m, but Terraria takes some accurate pointing and clicking... Steam Controller does well for a general mouse-based interface, but won't match the mouse for things like this.
Also, despite the general negativity of my comment, I'm actually open-minded about this and want to try it out again when controller support is brought in. Basically, I want to be wrong, and look forward to the nice surprise when I'm proven wrong. Until then, these are my expectations (and/or misgivings), so I'm not going to be disappointed if it turns out ugly.