I would have to agree with those who feel this is an odd choice. I would have thought that they would have brought back Fells if they wanted another TE, now that his contract wouldn't be an issue after passing through waivers.
For a TE Mulligan is strictly a blocker. In the past the Pats didn't hesitate to throw an OT out there in that position. (see Nate Solder.....the early years). So in the end, this is certainly a curious signing, but not very impactful either way, IMHO. But I doubt he survives to week 3