To unlock the It's Wintertime! trophy in Cities: Skylines, you will simply need to change the look of Chirper to one of the new winter looks that have been added with the release of the Snowfall DLC (although it doesn't require you to purchase the DLC).
To change Chirper, hold
to bring up the menu and select City info.
twice to the Chirper menu, press
to select one of the winter appearances.