What does this even mean.
Yeah, he's not a real person, he's a character in a video game. That's not the point, and it's quite weird you'd assume anyone meant otherwise. When you wrote "that list just proves how powerful he was", I didn't take it as if you thought Jon Irenicus actually existed in the past.
He's a plot device, used to move the plot forward. That's the problem with him. Apart from him making all those stupid decisions (employing Simon Haevarian lol) He's actually not even powerful in practice. The player never actually confronts him until the end and then when it happens he's just a run of the mill mage, no instakill spells, no Magic Missiles of Doom or anything. So when looked at as not a plot device, but as a character who needs to follow the basic rules that the protagonist/player must follow, his plans, his ideas and his strategy and decisions look very stupid. Of course as a plot device he's very very powerful, in fact he can't be defeated - the writer says so.
Meeting Jonnie is always controlled by the writer, except at the end. The player has no agency when encountering Jon, so the player has no power in those situations, only the writer does. Until the end Jon is like Kai Leng is in ME3, annoying and can't be beaten because the writer says so.
So the writer gives Jon inexplicable powers that are always convenient to the plot and Jon looks awesome until the plot stops, the game finally allows you to play against this all-powerful uber-mage, but this time with the rules afforded by the gameplay and yeah.. that's an anticlimax because he's usually wielding powers that can only be granted by the writer. When he has to rely ongameplay mechanics, turns, magic missiles and actually has to put his pants on one leg at a time like the rest of us, Jon is a pushover.