The Disputations blog has an interesting take (1 2) on theology in the book The Lion, the Witch and the Wardrobe, complaining that a part of this is "very bad art, and worse theology" and "cover-your-eyes awful theology" The central complaint is made against these lines:

The Witch knew the Deep Magic. But if she could have looked a little further back… she would have known that when a willing victim who had committed no treachery was killed in a traitor's stead, the Table would crack and Death itself would start working backwards.

which is the book's explanation as to why Aslan's sacrifice defeats the Witch. As to whether that passage works as art, opinions can obviously vary; I have no opinion to give here on that. But that the theology is somehow bad is an entirely different kind of complaint, and needs some examination.

C.S. Lewis's book is for the most part a Christian allegory. For the theology to be bad, I would presume that, in some way, the details of the allegory must misrepresent the Christian message in a bad way. From the grounds of his complaint, I would guess that Disputations also thinks that. He gives five grounds for complaint:

1. That traitors before the Stone Table is destroyed, and afterwards are thereby treated differently;
2. That the given reason why the Stone Table is destroyed could have worked for anyone, so why does it need Aslan;
3. That the Witch's power was already breaking anyway;
4. That Aslan tricked the Witch into her own destruction;
5. That the Narnia series has Aslan representing the Second Person of the Trinity.

Point 1 doesn't show the book changing the Christian message — the way that the destruction of the Stone Table (corresponding allegorically to the forgiveness of sins) changes things in Narnia, is matched by the distinctive change that Christ's sacrifice made in the real world (for example, see Hebrews 11, especially Hebrews 11:39).

As for point 2, the book says that the person who can crack the Table must be a willing victim in a traitor's stead. Exactly what is the representation in the book corresponding to willing, or to treachery is not made clear. Willing may signify more than just agrees-to-do it. Lewis may have had Romans 5:7-8 in mind, which has a high standard of willing in mind. Treachery (presumably the allegorical correspondence in the book to sin) is given an example by Edmund's betrayal of the other three. Treachery may not have been common in Narnia, nor true willingness to die in a traitor's stead — though Disputations thinks otherwise. This might point to a weakness in the background story, but not a weakness in theology.

Point 3 is true enough, but how is it bad theology? The Witch is weakening, but not broken, and her attempt to kill Edmund is designed to restore her power completely by ensuring that the prophecy (which needs two Sons of Adam and two Daughters of Eve) is not fulfilled. She takes Aslan instead of Edmund because she is greedy.

Point 4 may simply reflect an old theological position — that in some way the ruler of this world (here, Satan; there, the Witch) was tricked into killing Christ, and lost power that way. And 1Co 2:8 reflects something of the same idea — the crucifixion would never have happened if the rulers knew what would result. The allegory in Lewis's book follows this theology.

Point 5 I do not understand. Why should having Aslan as an allegorical Christ — subsequently being deepened into the rest of the Narnia series into having Aslan as an allegorical Second Person — change anything about the theology?

I'm not seeing any bad theology.