Goku being a Sorcerer with a single Monk level makes a surprising amount of sense. Sorcerers are typified by innate talent which could describe Goku's ability to pick up techniques surprisingly well and Ki as presented in-universe has a Mage Armor like effect. More broadly, Tulok tends to emphasize replicating the *feats* of a character over their exact canonical origin; Goku eventually reaches a level of Supernatural Martial Arts where those fantastic effects are better represented in D&D by spells.
Tulok's joke that the first sixteen levels of his Saitama build are the training the character did before the series started actually lines up pretty well with what ONE said is the core of humor in One Punch Man - that Saitama is a hero from the end of a battle manga being stuck at the beginning of it. In D&D terms that would be like a Tier IV hero taking on low-level threats. His wait to deal one-hit kills having a weakness against Legendary Resistance also fits since Saitama usually is late to fight major threats, after they defeated other heroes, with ease, to show our powerful and threatening they are. In D&D, it would mean they likely burned their Legendary Resistances to No-Sell whatever those heroes threw at them by the time Saitama showed up.
