IMO it’s rather that ‘Adventure Time’ and other cartoons were inspired by Don Hertzfeldt. Afaik Pendleton Ward was in CalArts right when Hertzfeldt’s shorts came out. ‘Adventure Time’ ran at pretty much the same time as ‘Regular Show’, and they had similar style and mood — not surprisingly, since Ward befriended J. G. Quintel (and Alex Hirsch) at CalArts. These three worked on ‘The Marvelous Misadventures of Flapjack’ together before launching their separate series.