The problem is 1 in 20 is far too frequent for the health of a long running game. I don't think DCCRPG is meant to be a long running game though.
If it really bothers you, a day after receiving a corruption effect, add a level check or some such. Roll a d10 and if it exceeds your level, the corruption fades, perhaps leaving a slight scar. So low level casters will be relatively corruption free. But sooner or later the corruption will stack up.
That's a really neat idea for the level check. It isn't the low-level cthulhoid wizards that bother me, though. Lots of games presume that players who are willing to cast cool spells are also willing to roleplay hideously ugly characters - see, for example, the "Nosferatu" clan in Vampire. I would be happy to have super-ugly wizards, if I thought that the rules were mathematically sound. What bothers me is that I suspect the mathematics are unsound.
No, what bothers me is that I get the sense that the game design is pushing multiple incompatible design goals on the player, and giving no means to resolve them.
E.g. Spells go up to level 7. One needs an 18 Intelligence to cast a level 7 wizard spell.
But the probabilities of the game dictate that it should be essentially impossible to make a 18 intelligence character who survives long enough to learn level 7 wizard spells.
The only way to make a high-level wizard is min-max and exploit the weak points. And that goes against page 10:
Using this method of highly random character re- sults, high mortality rates, and player choices as to which of their randomly-generated characters takes risks and which stays safe, you will find that you have a party of randomly generated characters in which the players have agency. There are essentially no op- portunities for min-max play
I can tell you, mathematically, how to min-max this. The judge has to allow players to generate 216 0-level mooks; one of them will have 18 Intelligence if the dice obey the laws of averages.
Of course, the spirit of the game suggests that if you don't get an 18 Intelligence in your first batch of four mooks, you simply have to give up on ever casting a level 7 wizard spell.
So WHY are level 7 wizard spells in rulebook? It's like the designer is saying, "I have a really cool level 7 wizard spell, and you *want* to cast it, and you roll very well for about six months of weekly play, I will let you cast it. But if, at any point during those six months, you roll badly, you lose everything."
At that point, it would be less alienating to say, "Level 6 and 7 wizard spells exist, but they are NPC-only. They are effectively godlike powers that are not possible for player-characters. Just like you can't play Cthulhu in Call of Cthulhu, you can't play a high-level wizard in this game."