To be sure, Joaquin Phoenix’s “not really The Joker” character already shares a few key elements with The Grinch.
Cindy Loo Who does manage to inspire Grinch to flip his frown upside down by the end of that tale, but Joker contained no such happy ending, with the Arkham-imprisoned Arthur still nowhere near his right mind.
Second, they commit sins against others without really caring how the other people will be affected by their actions.
For one, they’re both very lonely characters with very strong opinions that they physically enforce.