Dewey rarely snitched on his brothers because doing that would be less thrilling for him than participating in whatever shenanigans they were up to. Besides, the perks that came from grassing on his siblings were probably very meagre. In the "Christmas" episode, he along with Malcolm and Reese were caught by Lois smashing up household items. Even if Dewey didn't personally damage any objects, he was there and clearly reveling in the destruction. So Lois would punish him along with them. In "Red Dress", again, even if he didn't burn the dress himself, Dewey might still have known who did it, as far as Lois was concerned. Just concealing it from her would make him as guilty as Reese or Malcolm. Besides, what makes you think a six year old is incapable of arson. In Lois' flashback in "Lois Battles Jamie", a two year old Francis was about to set his teddybear aflame before his mother stopped him. For such a small child, he looked like he knew what he was doing. Or perhaps Lois over-reacted? Well, their all capable of something.
If the point your trying to make is that Dewey gets caught doing a lot of forbidden activities, then I should point out that Malcolm often got caught doing something he shouldn't far more than him. Academic skills or no, Dewey at least has more common sense.