I certainly think the band should try and look like they're having fun, not necessarily smiling though. I know from my own playing (for morris) that it is hard to smile when you're concentrating on playing but my enjoyment of a ceilidh is enhanced by the band having (or seeming to have) fun too.

Whapweasel are, in my opinion, almost as much fun to watch than to listen or dance to. But they have the advantage of seven members who are not all playing together at the same time so they can have some fun on the stage. Smaller bands, such as a trio like the Bismarcks are working all the time so don't have that possibility.

Also the instruments and amplification makes a difference too: how do you smile when playing a flute, whistle or sax? How much freedom do you have to wander around the stage?

So, try and look like you're having the fun that you are experiencing. It'll go down well and people won't think you're being mardy or bored.