As someone who has lost 2 loved ones, one for Senile Dementia, and other for Alzheimer, your comment is quite frankly rude and distasteful, it is 0% funny to see people you loved and cared slowly losing themselves, and becoming a husk of their former selves, until they died, its heartbreaking and soul crushing, think your words carefully next time, pal
I'm sorry for you, but you don't have any moral ground to say want someone can or can't say. I too have lost people from Alzheimer, yet the comment was funny and I joked about it too. Because my Grandpa joked about it all the time at first.
So, if it's rude for you, 100% fine, you can and have the right to think it's rude, but you don't have any right to say what's funny or not. It's not funny FOR YOU, I get it and it's okay, but I'm in the same situation that you are and I think it was funny and help laughing at these matter.
If I am to say the same message that you are but changing it saying that I a post like that makes me laugh and remember my time with my Grandpa and I'm really glad of it, you'll probably have this whole conversation about why you think it's wrong and blablabla but neither of us is in the right or wrong.
Just that, if you need to point every single thing on the internet that YOU find offensive, you're not cut for it. Just, think it's rude, shake your head and that comment and past to the next.
....
Or you can... forget about it.