Acme's will isn't quite a red herring…because it ends up being an important factor in the plot. But like the ink it's written in, the will disappears for most of the movie, only to reappear at the end when you least expect it.
Before he's killed, here's Marvin Acme explaining his gag to Eddie:
MARVIN: Now calm down son, will ya? Look, the stains gone. It's disappearing ink. No hard feelings I hope?
Acme's a guy who jokes all the time. But perhaps he should have taken his will seriously? If Acme had written his will in normal ink like a normal person, the drama surrounding Doom's takeover and the near-eradication of Toontown wouldn't have happened.
It's great to have a sense humor, but you need to know when to keep it in check. Sometimes taking a joke too far can really hurt someone…even if the main casualties are just a back of psychopathic weasels and a Toon gone bad.