As one reviewer for Uncle Grandpa mentioned, and I'd have to agree, it has a crazy 90s cartoon kind of vibe. Some of the strange, disconnected scenes remind me of just how bizarre and surreal "Rocko's Modern Life" and "Cow and Chicken" could get; the art style kind of reminds me of the bright, scraggly lines style of "Whatever Happened to Robot Jones?"; even the character designs have more linework, facial expressions, and body types more comparable to 90s toons than modern toons.
Part of the show's appeal to older audiences might just be how much the art style could be considered a hallmark to the 90s, but that aside, I personally think it's a spectacular addition to Cartoon Network's line-up. Regular Show and Adventure Time started the trend for strange, surreal, and abstract elements to be popular staples of current animation trends, which is probably why Uncle Grandpa was given a chance at all. Though, the show stands very well by itself, a very unique and different diamond in the rough. A large part of that is the art style, but it's also different in story execution, tone, and audience.
Nothing about the show takes itself seriously. The show takes cartoon logic to levels that would make Bugs Bunny's head spin, but places it in a genre comparable to Cow and Chicken; it's so mindless it takes suspension of disbelief to the most extreme threshold, getting the viewer to either delight in the absurdity or groan and grit their teeth as they try to make sense of something as nonsensical as an Alice in Wonderland un-birthday party. If you're the former, Uncle Grandpa is a fun stress-reliever! Personally, I love it. Uncle Grandpa is a fun and unpredictable character, one that you would classically associate with cartoons in the sense that a toon is wacky and unpredictable. Every episode is something different, playing out in such a way that you'll never predict what's going to happen next, or even how each 'plotline' will get resolved.
One thing I really want to note about Uncle Grandpa is the tone and intended audience. The lighthearted and fun tone contrasts Adventure Time and Regular Show with their more developed, character-driven, and serious narratives. I'm not calling this a bad thing; I honestly invite the notion of a few mindless, crazy cartoons being stacked up in CN's current line-up. Not only that, but Uncle Grandpa really is a general audiences kind of show. It's innocuous, with the main focus being to present an imaginative narrative. Uncle Grandpa may very well be remembered by some as fondly as Cow and Chicken for 90s rugrats, if only for being one of the stranger and off-the-wall shows on Cartoon Network in this time.