In the Movie, Grumpy Cat's Worst Christmas Ever, Grumpy Cat, Near the very end, Expresses some slight happiness at the girl whose name I forget. Basically, Grumpy Cat can pull off the lightest of smiles, But only on Christmas when she is very happy. Merry Christmas, Tartar Sauce. Merry Christmas. And for those who say that this movie does not fall in line with the Grumpy Cat canon, Then you're just denying the truth by now.
(IRL Tartar Sauce is a nice kitty with feline dwarfism and an underbite, So she looks grumpy but is nice. I've found a few where she genuinely looked happy. )