Aight, I enjoy the joke too.
However! I encourage people to remember that grandpa joe is not a faker in the world he's from!
Since the movie is what most peeps remember, and where the memes usually come from, the first thing to remember is that it's a musical.
Musicals, by the established rules of the overall genre, do not reflect reality at all times. Even mostly dramatic musicals like Man of LaMancha break some reality in order to function as musicals. Take the scene with the ruffians and "Dulcinea" as an example.
Second, the movie. Willy Wonka and the chocolate factory is essentially a fantasy piece. You've got the Oompa Loompas as prime evidence of that. Orange skinned humanoids that do not exist in the real world (jokes aside). Many things in the chocolate factory break the laws of physics or otherwise bend reality. There's geese laying golden eggs, ffs.
Third, the theme of the movie isn't actually torturing children. The theme of the movie is the redemptive and uplifting power of dreams. That's achieved by the journey of Charlie getting his golden ticket and everything in his life getting better.
Grandpa Joe hasn't been laying there in bed faking it (though, in movie, there's never anything about the grandparents being unable to move or walk at all, they're just frail and weak).
He is in his eighties or nineties.
What gets him up and dancing isn't that he was faking and forgot to, it's joy.
GJ is transformed by joy, by happiness. His grandson has, through luck or destiny, gotten the golden ticket to a brighter, better life! This doesn't trick Joe into forgetting his infirmity. It gives him the joy to overcome it.
Joe's transformation, rejuvenation, is because he is so filled with joy that his grandson will have a new life, that it changes him into the grandfather he wished he could be. Don't forget that he had sacrificed his one real pleasure to give Charlie a chance at that.
But, look, I know that the grandpajoehate is ostensibly a meme. It's a joke poking fun at the very musical rules that allow a bed-bound person to magically be cured in the first place. But it never acknowledges the fact that his spontaneous rejuvenation is magic, and that the magic is the magic of love.
In a cynical world, we believe that love is not transformative because the real world grinds us down. But love can be transformative for us too. We just have to be willing to let it work.