N.M.
I think when clerks and strangers ask those kinds of things it's just to make small talk. If you don't want to do the smile-and-nod, since she's only 2 you can answer for her, simply and sweetly "oh, we don't do Santa (Christmas), but isn't the weather lovely today?"
We celebrate Christmas but not the whole Santa bit (like one of the other moms said). And now that my kids are older (6 and 8) and understand a lot, they know how to do the smile-and-nod and talk later about how some people do Santa and others don't and how not to spoil other people's fun with it. They know not to say "We know Santa isn't real!!" because there might be Santa-believing kids around.