I personally didn't care. His celebration was well within the bounds of good taste. Without knowing what's in his head, from what I know if his personality (and I mean that in a good way), I would have found it entirely disingenuous if he didn't celebrate at all. I actually find it to be somewhat self important when guys make a big show of not celebrating when they score against a former team, particularly in the case of a guy like Mullins who played here for 1+ seasons. I know, though, this is a topic many people have a lot more energy around than me. Its just not one of my hot buttons.
But, on the subject of celebrating against former teams, I have to bring out my favorite Emmanuel Adebayor clip for a while back. That, was probably over the top, though I'll admit it made me laugh at the time.