Grammar structure of "你看，Sally 穿上传统服后有多漂亮“
你看 is a proclamation
Sally is the only subject in &quot;Sally 穿上传统服后(她)有多漂亮.&quot; The pronoun '她'(she - referring to Sally)  is omitted. You don't need to repeat the subject in the single sentence

&quot;你看, Sally 穿上传统服后有多漂亮&quot;
Look! How pretty Sally looks after (she) put on a traditional dress.

if 后 is removed, the sentence would become &quot;你看, Sally 穿上传统服有多漂亮&quot;

Look! How pretty Sally looks (when she) puts on a traditional dress.