Is a circle of fifths that goes backward (eg. D G C F Bb) still considered a circle of fifths progression?
Typically, a circle-of-fifths progression is a progression that moves by descending fifths. Thus, your progression is a standard circle-of-fifths progression!
The reason descending fifths are so common is due to the dominant-to-tonic relationship between two adjacent chords. In your example of
D G C F,
D is the dominant to
G is then the dominant to
C is then the dominant to
F, and so on. One of the many reasons this dominant-to-tonic relationship is so convincing is that each chord contains the leading tone of the next chord.
G contains the leading tone to
C contains the leading tone to
Ascending fifths are much harder to pull off convincingly because it lacks this dominant-to-tonic relationship, and consequently each chord lacks the leading tone of the next chord.
However, in the ascending-fifths progression, each chord has the leading tone of the chord before it! So if any fifth progression is "backwards," it is the ascending one.
The circle of fifths progression is I - IV - VII - III - VI - II - V - I.
The circle of fifths progression is commonly a succession through the seven diatonic chords of a diatonic scale by fifths downwards, including one progression by diminished fifth, (in C: between F and B) and one diminished chord (in C major, Bdim), returning at the end to the tonic.
In major keys, it will be I - IV - vii° - iii - vi - ii - V - I. In C major, it will be: C - F - Bdim - Em - Am - Dm - G - C.
In minor keys, it will be i - iv - VII - III - VI - ii° - V - i. In A minor, it will be: Am - Dm - G - C - F - Bdim - E - Am.