Imposter Mech does not copy any subtypes related to a card type that it lost. For example, if Imposter Mech enters the battlefield as a copy of a 1/1 Human Soldier token, it is neither a Human nor a Soldier. It won't gain those subtypes if it is later crewed.

If Imposter Mech enters the battlefield as a copy of a creature that was also a planeswalker, it loses both the planeswalker and creature card types. Its controller may activate any loyalty abilities it now has as normal, but it notably cannot be attacked and it does not lose loyalty counters when it is dealt damage.