how does Twain express his skepticism towards men who claim to be "reformed" and those who push to "reform" others?

I think this has much to do with Pap swearing to the judge that he will change his ways, stop drinking and become a god fearing Christian if he can take custody of Huck. Twain shows the self-righteous judge to be naive, only believing Pap to further his own belief in sin and repentance. The reformer gets more out of the deal than the person reforming; it validates the reformers powers of perception and "healing". The irony or truth is that that Pap tearfully repents and then gets drunk again.