People have often punned about Grimm's fairy tales being very grim. For example, TV Tropes has the trope Grimmification about tales being made more grim. (The Brothers Grimm didn't engage in Grimmification - they didn't need to. In fact, they Disneyfied.)
Are the two words "grim" and "Grimm" etymologically related?
The English edition of Wiktionary does not have an entry for "Grimm", nor does the Online Etymology Dictionary.
Wiktionary states that grim came from Old English, which came from Germanic. And Grimm is used as a surname in Germany.
Grimm has an entry in the German Wiktionary, but even with Google translate I can't tell if it's talking about a name, a noun, or both.
One person I talked to about this said that "Grimm" meant "hooded", which would explain "The Grim Reaper" (apart from it only have one "m"). Is that true?