Beauty is in the eye of the beholder
If using 西施, (as a general / traditional indication of physical beauty), is not comfortable to you, then you can say, 情人眼里出美人 or 美在情人眼.
BTW, 旁觀者 means &quot;bystander&quot;, i.e., someone who is an &quot;objective observer&quot;, whereas &quot;Beauty is in the eye of the beholder&quot; denotes a &quot;subjective assessment / opinion&quot; by an involved beholder.
So, 美在旁觀者的眼睛, (which is unnecessarily verbose and awkward) could be &quot;idiomized&quot; to 美在情人眼.
However, if you are looking for a &quot;generic&quot; phrase that applies to both persons and objects of interest, then perhaps 美在各眼中？which takes away the narrower application to 情人 only.