Awful - just awful.
While it can quickly be explained away that Ben altered Von Doom's recipe for chicken soup rather than a pandimensional thingamabob (like he could have told the difference anyway), this really is a silly thing for Ben to feel guilty about.
The notion that this is why he became the Thing is just as silly. If you want fanfic to explain it, then why not just say that Ben became the Thing because he feared the Cosmic Rays (Fantastic Four #1 (1961) page 9 panel 1) and so he made sure his
spacesuit had heavier shielding in it than the others. Once they got blasted, he was hit with a smaller dose because of the shielding. The others maintained full control over their transformations over the years because they received the full dose. Had Ben wore the same kind of suit he could have changed into the Thing at will from the very beginning.
So for all the times he's given Reed grief over not being able to cure him, Ben's known all along that he's responsible.
So there, the same type of confession that does not lessen another character at all.
And Ben's timing - ugh. He's never left behind this confession before despite the billion times he's walked into certain death situations. Why now? Why burden two kids who were just abandoned by their family with this? This is a dickish thing to do - and Ben's not a dick.
But since his confession is now on the printed page, how long before Sue and Wolverine go back in time to kill Ben for creating Doom? (that is the kewl way of dealing with things now right?)