In the last part of Mirror Image’s description, it says (emphasis mine):
A creature is unaffected by this spell if it can’t see, if it relies
on senses other than sight, such as blindsight, or if it can perceive
illusions as false, as with truesight.
Witch Sight is not a sense to be relied on – rather, it augments the sense of sight:
You can see the true form of any Shapechanger or creature concealed by Illusion or Transmutation magic while the creature is within 30 feet of you and within line of sight.
So the remaining condition is whether or not it allows perception of “illusions as false, as with truesight”.
Truesight is described as follows:
A creature with Truesight can, out to a specific range, see in normal and magical Darkness, see Invisible creatures and Objects, automatically detect visual illusions and succeed on Saving Throws against them, and perceives the original form of a Shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal Plane.
This description explicitly gives Truesight the feature “automatically detect visual illusions” in addition to the ability to perceive “the original form of a Shapechanger or creature”, the latter of which is almost a word-for-word copy of Witch Sight.
In summary: Truesight sees through Mirror Image because it can “perceive illusions as false”, due to its ability to “automatically detect visual illusions”. Witch Sight can only “see the true form”, and thus cannot see through Mirror Image. If a Shapechanger used Mirror Image, Witch Sight would see duplicates of the Shapechanger’s true form, but would still see the duplicates.