#A simulacrum is a creature

You shape an illusory duplicate.... The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature.

Since the spell says the duplicate is a creature, you treat it as a creature for all relevant effects, including the effects of true polymorph.

A simulacrum is a creature

You shape an illusory duplicate.... The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature.

Since the spell says the duplicate is a creature, you treat it as a creature for all relevant effects, including the effects of true polymorph.