He hath cast me into the mire - That is, God has done it. In this book the name of God is often understood where the speaker seems to avoid it, in order that it may not be needlessly repeated. On the meaning of the expression here, see the notes at Job 9:31.
And I am become like dust and ashes - Either in appearance, or I am regarded as being as worthless as the mire of the streets. Rosenmuller supposes it means, "I am more like a mass of inanimate matter than a living man."

(n) He hath cast me into the mire, and I am become like dust and ashes.
(n) That is, God has brought me into contempt.

He hath cast me into the mire,.... As Jeremiah was literally; here it is to be understood in a figurative sense; not of the mire of sin, into which God casts none, men fall into it of themselves, but of the mire of affliction and calamity; see Psalm 40:2; and which Job here ascribes to God; and whereby he was in as mean, abject, and contemptible a condition, as if he had been thrown into a kennel, and rolled in it; and he speaks of it as an act of God, done with contempt of him, and indignation at him, as he apprehended it. Some Jewish writers (e) interpret it, "he taught me in the mire", or "it taught me"; his disease, his ulcers taught him to sit down in the mire, or in the midst of ashes, Job 2:8; but though this reading might admit of a good sense, as that Job was taught, as every good man is, many useful lessons in and by afflictions; yet it seems to be a sense foreign from the words:
and I am become like dust and ashes; a phrase by which Abraham expresses his vileness, meanness, and unworthiness in the sight of God, Genesis 18:27; Job, through the force of his disease, looked like a corpse, or one half dead, and was crumbling and dropping into the dust of death and the grave, and looked livid and ash coloured; and even in a literal sense was covered with dust and ashes, when he sat among them, Job 2:8; though here it chiefly respects the miserable, forlorn, and contemptible condition in which he was.
(e) Vid. Jarchi & Bar Tzemach in loc.

God is poetically said to do that which the mourner had done to himself (Job 2:8). With lying in the ashes he had become, like them, in dirty color.