Despite Walt's supposed intelligence, the plans he concocts are markedly ridiculous, such as impersonating Mr. Panucci to elicit information from Fry, and having the trio pose as owl exterminators to get the anti-backwards crystal when he knows absolutely nothing about owls or their extermination. They only work because it's Fry the plans are mostly directed at.
Another example of Walt's childishness is shown in the episode Mother's Day. When he learns that Professor Farnsworth is the man who broke Mom's heart, like a child whose parents have separated, he assumes that bringing Farnsworth and Mom back together should be a simple task that will make her happy and thus everything will be alright again.
Larry is a minor character on Futurama. He is the sniveling middle child of Mom's three sons. His brothers are Walt and Igner. Larry is the middle child and is somewhat less intelligent than Walt, but nonetheless the most stable of the three. Unfortunately he is entirely submissive to his older brother and mother, agreeing with everything thеy say. He apologizes for almost everything that goes wrong, even when it's not his fault, and thanks his mother for insults and slaps. Larry is struck the most often of all of Mom's sons, and not as one would think, Igner. This is probably because the writers decided that having Igner struck for his immature statements and infantile behaviour would be a lot less funny than striking Larry for no reason.
Of the three sons, Larry seems to be the most normal, and has been shown trying to get a date in the series and in the comics. In Put Your Head On My Shoulders he is seen standing outside Bender's dating service for the miserably lonely. In issue #34 of the comics he is shown using speed-dating, and talks briefly to Amy, who comments that he seems a nice guy, but she already has a boyfriend.
Igner is the youngest and most pitiful of Mom's three sons. His older brothers are Walt and Larry. Like them he owns a 0.1% share in MomCorp. His intelligence is extremely low, and he often displays the symptoms of mental retardation. He speaks in a stereotypically moronic voice, and is often found saddled with the worst tasks by his brothers, being bullied into wearing Mom's bra, and playing the part of Pamela Anderson's body for her head in a jar. His immaturity means he gets disgusted easy, and he has a tendency to repeat what he hears without full comprehension, for example: in Bender's Game, Igner repeats the phrase "We're owl exterminators" twice, and pronounces exterminators wrong. In Bender's Game, Igner helps Fry, Leela and Farnsworth escape his pursuing brothers, by denying that he had seen anybody to Walt. However, while everybody else was going along with the Dungeons and Dragons/Lord of the Rings parody, Igner's alternate reality persona got confused and pulled out a lightsaber, and started parodying Star Wars.
It was revealed in Bender's Game that Igner is the son of Professor Farnsworth, making him the distant nephew of Fry and half-brother of Cubert, while his brothers have so far been assumed as the sons of Mom and Wernstrom.