The titular character being everyone's uncle and grandpa brings along some Unfortunate Implications, but when you realize what a huge Reality Warper he is, it makes (a little) more sense.
Alternatively, considering how much of a Cloudcuckoolander he is, it is possible that it's simply a title he came up with without thinking about the implications.
"Charlie Burgers" is the only episode in which the kid of the week doesn't seem to benefit at all from Uncle Grandpa's help. While there are several episodes where the kid doesn't get what they originally wanted, most of them at least end with the kid learning some kind of valuable lesson, but Charlie Burgers doesn't seem to gain anything. "Charlie Burgers" is also the only episode where the kid of the week is a dog rather than a human. In other words? It's a "Shaggy Dog" Story.
More Irony than Brilliance, but Pizza Steve's constant bragging. Uncle Grandpa is a time-travelling Reality Warper and has all kinds of powers, Belly Bag is a Bag of Holding who can provide access to multiple dimensions and contains a whole bunch of other magical items, Mr. Gus is over a billion years old and strong enough to work out by squashing buildings, Giant Realistic Flying Tiger can fly, shoot rainbows, and fight off a cemetery full of zombie animals... but Pizza Steve is the only crew member of the RV who hasn't demonstrated any extreme skills or powers. Perhaps his bragging is an attempt to compensate. The episode with his diary outright states that he wishes he was more book smart and classy like Gus.
The latter half of "Bad Morning" seems to imply this, when Uncle Grandpa finds him sulking in a diver's helmet.
Uncle Grandpa: *gasp* Pizza Steve! What are you doing down here? Pizza Steve:....this is where I come to cry. Uncle Grandpa: WHA?! Pizza Steve: I said, uh....this is where I come to be a cool guy!
A bit with Aunt Grandma: her motivation is that Uncle Grandpa causes a tiny amount of damage to her science fair project on accident that made her get second place in a science fair competition. While it's understandable to be upset at this, it's still no reason to dedicate your life to ruining someone's career. Think about why Uncle Grandpa says he visited her: to teach her she needed to relax and have fun once in awhile. THAT was the problem he was trying to help her with, not her science fair project. Given the level of perfectionism this implies Aunt Grandma had, it's kinda easy to see why Uncle Grandpa thought it was the most pressing problem she'd had at the moment. It's just Aunt Grandma refuses to accept that it was a problem even when it's pointed out to her face. This also plays into her more practical methods: she fails to see the point that Uncle Grandpa has in his adventures: to make them fun and perhaps even teach the kid an important lesson if what they want isn't what they need. This is because she was in the latter camp, and failed to learn her lesson.
It also explains why she snapped: In her eyes, Uncle Grandpa failed to help her, though for her it was for the science project. However, when he explained he was trying to get to lighten up, she ends up snapping. Then you realize that she is still right in that he failed to help her in helping to have her learn what he was trying to teach all along.
Uncle Grandpa sending Riley to a demonic party because he snuck off to a party when he was grounded seemed a bit harsh, but then when you remember his encounter with Aunt Grandma and her backstory, so it makes sense that UG would really want Riley to learn his lesson, so he doesn't end up like her.
Unfortunate Implications, but when you realize what a huge Reality Warper he is, it makes (a little) more sense.