After thinking of it for a while, I'd like to shed some poetic light on it.
1. Similiar usage
I got reminded of another similiar usage of the "de-" prefix in another Ted Hughes' Crow poem, called "Crow's Account of St. George":
[...] He refrigerates an emptiness,
Decreates all to outer space [...]
The word "Decreates" is Hughes' neologism, using the "de-" prefix in a unique, poetic sense. One can suggest many interpretations of this usage, but surely enough, since this is a neologism, those interpretations would concentrate on the two parts of the word: The prefix (de), and the verb (create).
So, I think we can refer to the phrase in question in a similiar way: We need to concentrate separately of the two parts of the word "deformed": The prefix (de), and the verb (form).
2. Suggestion: A Biblical Allusion
If we search for "formed against" in Google, the first result is the last verse of Isaiah 54 (all of the following citations are taken from King James version):
No weapon that is formed against thee shall prosper; and every tongue
that shall rise against thee in judgment thou shalt condemn. This is
the heritage of the servants of the Lord, and their righteousness is
of me, saith the Lord.
So we can see that tongue and a formed-weapon coincide, and so we can understand the phrase in question as a poetic arrangement of this metaphor.
But what does Isaiah 54 has to do with "The Contender"?
3. The Servant Songs
Well, though it's not considered as one of the four Servant Songs, Isaiah 54 is the chapter that follows the last Servant Song - which is a part of Isaiah 53, arguably the most known Old Testament chapter that - according to Christian discourse - is a prophecy of Jesus. Isaiah 54 itself is also referred to in Christianity as speaking of Jesus.
The four Servant Songs, especially the last one, depicts lengthily the suffering of "the servant", and suffering is also the main subject of "The Contender".
Ok, so all is good and well, but what's "mouths deformed against stone"?
4. Wrapping It All Together
In the third Servant Song, the Servant's face is depicted as a flint:
For the Lord God will help me; therefore shall I not be confounded:
therefore have I set my face like a flint, and I know that I shall not
[Relatedly, in Isaiah 54:11-12 we read:
O thou afflicted, tossed with tempest, and not comforted, behold, I
will lay thy stones with fair colours, and lay thy foundations with
And I will make thy windows of agates, and thy gates of carbuncles,
and all thy borders of pleasant stones.
So, to expand on what @giorgi rcheulishvili have correctly simply put, we may suggest that the women in the poem approach the man with good intentions, maybe trying to convince him to leave the suffering behind it, but their words are hitting a stone-faced man who considers their words as weapons trying to deflect him of its passion (pun intended).
5. A Side Note
Isaiah 54:5 talks about "The Redeemer":
and thy Redeemer the Holy One of Israel
Considering that, it's pretty clear that the poem's title is quite ironic.