Average Joe from Accounting got sucked into the world of DnD.What class would he be?

Lvl 0 Commoner

CLERiC

Sounds like clerical work to me.

Wizard probably. No spells but if you can learn the mind numbingly boring shit you need to learn from accounting you can learn magic fucking missile from a scroll

Depending on the insanely long title of the isekai, whatever the plot needed them to be.

If this is not a rhetoric question and you really want to play this, I'd advice to look into other pnp games. E.g. Shadowrun or The Dark Eye are much better suited to play non-superhero humans (as are many other games as well).

My current character wasn't isekaied, but he was an accountant before he became an adventurer. He worked for a dragon. He's a rogue now (and a bit of a loan shark). He does have lock picking skill though, because it was necessary for inventorying the dragon's horde

A rogue, but he would be so bad at anything rogueish he would have to learn a trade to survive and only ever live out his rogue-ishness through knocking some decimals off a price of an item.

The spell options and classes in Palladium, Rolemaster and Swords and Sorcery had a lot more flexibility. For the most part any class can learn some magic. For fighters it's more observation or stealth type stuff unless it's barbarian type classes that may get more bard type skills and there are like 5 different types of thieves/rogues/burglars, etc. like most other classes. When I ran a group we used a lot of different rule books. Some offered better detail for hit location and armor by location, some had better control of weapon type and how it's used like bash, pierce, slash, and as noted above, the magic in other rules had so much flexibility. I gave my players a lot of options as to how they built their character and having 4 or 5 rules books of different skills, classes, races, etc really opened up variety. An accountant may be a really good burglar with attention to detail and noticing patterns

If you really want to play this, you'll need to look at other pnp games. Shadowrun or The Dark Eye have much better mechanics for playing a non-superhero human.