Rocko and SpongeBob have a lot of similarities - I think even people who have only heard of Rocko in passing have some kind of awareness of this, hell whenever I try to describe Rocko to people who aren't into 90s cartoons I say "the social satire of The Simpsons combined with the slapstick of SpongeBob" - but it is kinda insane just how many similarities they have. Almost every main cast member you could find some kind of SpongeBob parallel to, and there's tons of plots that were reused in SpongeBob, ranging from extremely simple plots (friend loses their home, protagonist takes them in and loses their patience; character is given a cop-like role while lacking the responsibility, leaving someone to get arrested) to extremely specific and wild ideas (character drawn without a nose gains an artificial nose, and is horrified to learn of bad odours; protagonist's childhood bully is said to visit them after a set amount of time, after a lot of worry they turn out to be completely harmless)
That being said, these plots are all handled different ways - for example SpongeBob's bully BlackJack had shrunk over the years and is now too tiny to hurt anyone, while Rocko's bully Dingo had become a pacifist monk since their last encounter. When Patrick got a nose, he tried to violently destroy everything that radiated a bad odour, while when Bev got a nose she took issue with Ed's BO specifically, causing a marital issue.
----
At the moment, the Mario RPGs are lacking playable representation. While these beloved games are represented in other areas, such as the Paper Mario stage (which itself is ported from Smash 3DS), having this notable section of Mario's legacy be present via a fighter would be a neat addition. With Geno specifically, not only would we be representing the first-ever Mario RPG, but one that is highly revered within the fanbase, with many calls to bring it to the NSO SNES game selection since the service was introduced.
Geno also has quite a lot of moveset potential - While he only uses a few special attacks in SMRPG, his status as a Swiss army knife of a doll could give him a lot of interesting moves as he uses parts of his body as weapons. A lot of creativity could be used in designing his kit (although canon abilities such as Geno Whirl are still a must).
Finally, Geno has been a highly-requested character in Smash for many years, with his popularity being somewhat acknowledged by the actual Smash games with a Mii Costume being introduced in Smash 4 and a Spirit in Ultimate. Despite only having one major appearance to his name, Geno has captured the hearts of many fans, and that in itself is commendable.