I've got to take the time to give this at least one positive review, since the other guy has misunderstood it. No, it is not a perfect 1:1 recreation of D&D. It takes a bit from 5e and a bit from 3e, it has faith bonuses more reminiscent of roguelikes. It has some things that definitely look like house rules, albeit extraordinarily reasonable ones. We aren't presented with a rulebook, and some of the numbers may be fudged or silly, but none of it is beyond the bounds of common sense. This is the callout portion of the review saying to disregard the other commenter who has probably only ever played 5e and doesn't remember half their class features anyway, someone who likes 'the narrative' and made a habit of ignoring mechanics.
And that is where Isekai Munchkin shines. Maybe not the brightest, but it does utilize many combos that fit the vibe. An enlarged octopus octo-wielding daggers each with a different poison? A very sensible TTRPG tactic. It falls short because the author does tend to use these as entertaining gimmicks. The octopus does not become a staple of their fighting style(although it remains as a mascot). Many of the conflicts feel episodic, the lessons and tactics used being unlikely to show up again. Even so, wight level drain scamming, and converting religion for maximum benefit, all have the charming feel of a munchkin very new to the game.
The plot so far is average and decently executed so far. Nothing particularly special, but it stays mostly on task in a fun way. The MC is off to find/save his sister inadvertently gathering more and more evil compatriots along the line. The story stands to gain something by committing a little harder to some of the grim points. Maybe it's because the last few things I've read weren't shy with gore, but it feels a little too clean for a campaign steeped in evil. The shapeshifter was a step in the right tonal direction, so I'm hopeful.
The characters themselves have a little bit of depth, not fleshed out to make them stunning, but they're not simple caricatures. The MC expresses mild hesitation working with 'Evil' to save his sister, but not really that much. The paladin is stubborn in following her code.but willing to bend rules to do good. Yet we don't get a whole lot of characterization outside of what's directly relevant. Very like a wargaming group, but not so great when reading a narrative.
In summation: A worthy gimmick read for TTRPG players, and if the author continues to improve even slightly, the mostly sufficient story and characters will be outright good for anyone to read.