This is a good question. Under normal circumstances making of such an image was forbidden:
Exodus 20:4 “You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth.
However in this case God commanded Moses to make it not as a god or idol, but as a representation of the Israelite’s own punishment.
Num.21:4 From Mount Hor they set out by the way to the Red Sea, to go around the land of Edom. And the people became impatient on the way. 5 And the people spoke against God and against Moses, “Why have you brought us up out of Egypt to die in the wilderness? For there is no food and no water, and we loathe this worthless food.” 6 Then the Lord sent fiery serpents among the people, and they bit the people, so that many people of Israel died. 7 And the people came to Moses and said, “We have sinned, for we have spoken against the Lord and against you. Pray to the Lord, that he take away the serpents from us.” So Moses prayed for the people. 8 And the Lord said to Moses, “Make a fiery serpent and set it on a pole, and everyone who is bitten, when he sees it, shall live.” 9 So Moses made a bronze serpent and set it on a pole. And if a serpent bit anyone, he would look at the bronze serpent and live.
Here God specifically required the people to “look” at it, not to worship it. 700 years later, when the Jews later did start to worship it, king Hezekiah had to destroy it.
2 Kings 18:4 He removed the high places and broke the pillars and cut down the Asherah. And he broke in pieces the bronze serpent that Moses had made, for until those days the people of Israel had made offerings to it (it was called Nehushtan).
Nehushtan sounds like the Hebrew for both bronze and serpent
By the same token in John 3, John does not say that Christians are to “worship” Jesus
John 3:14 And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, 15 that whoever believes in him may have eternal life.
Likewise Christians “look to Jesus” but are not instructed to “worship” Jesus as God (there is also a second Greek word, often translated ‘worship’ in English Bibles, which means “bow down” as to a king — people did bow down to Jesus)
Heb.12:2 looking to Jesus, the founder and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God.
Although it seems strange, even offensive, for Christ to be compared (by the apostle John) to the serpent on a stick, nevertheless, that is part of what Christ’s sacrifice was, lifting up the weakness of human nature which Jesus had inherited, via his mother Mary, from his ancestor Adam.
Romans 8:3 For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh,
2Co.5:21 For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
See also Peter Watkins, The Cross of Christ