It's a ruse, She actually loves it almost as much as we do but she's over-compensating with disdain to keep from being "outed" And I have absolutely no confirmation of this, just surmising
If you listen closely, when Joy is tickling Sherri, Elisabeth can be heard saying "Can we get H.R. down here?" while she is wincing from watching Sherri getting tickled. (how many times have any one of us played it off, and made casual, nonchalant (*spelling?*) remarks or comments to try to deflect or mask our genuine interest in the situation, like when tickling is going on in a real-life context, like in the vanilla world and it makes us slightly uncomfortable?) I can personally attest to it, even in my youth back in school and such.
this is one to ponder