dnd 5th – What do you get when you create a simulacrum of a real polymorphic creature?

True polymorph said:

You turn the creature into a different creature […] The target's game statistics, including mental ability scores, are replaced by the statistics in the new form. He keeps his alignment and his personality.

The creature is now a completely different creature, up to the stats block.

And travesty do not care what a creature statistic in use to be, he copies the statistics currently for the creature:

the illusion uses all the statistics of the creature that she duplicates

So, you will get a dummy of the form that you used true polymorph turn into. Of course, the form meets all the requirements for travesty.

travesty do not copy the effects of active spells on the creature so that the simulacrum never returns as the non-polymorph creature.