The topper which sits at the "crown" of a Christmas tree comes in a variety of forms, most common of which is a star (representing the Star of Bethlehem) or an angel, along with other thematic ornamental decorations like Christian crosses, white doves, etc.

And then there's the tree topper which comes in the form of an actually living, breathing entity.

They could just be a living version of any of the above-mentioned ornaments (or even just simply bears a resemblance), but more commonly they may actually be just be a full human/human-equivalent that is (oftentimes literally) roped into the role. Primarily a gag at the subject's expense, assuming they haven't willingly subjected themselves to this for whatever reason. ...