Is there any recorded use of the phrase, "Beauty is in the eye of the beholder" being used in a commercially available D&D campaign or any rulebook of any edition, specifically to reference (secretly or overtly) a beholder that appears in the course of the campaign? What's the earliest instance?
The earliest reference is 1987
First time I heard it referenced in Dungeons and Dragons was in the 1987 Saturday morning cartoon.
Not really published though and not exactly canon.
The earliest reference is 1990
Not exactly a campaign but an early issue of Dragon magazine had a “cute” drawing of a beholder on the cover. The issue’s tagline is “Beauty is in the eye of — oh, skip it.” It was their 1990 April issue, #156:
(This issue is in the tradition of April issues of Dragon being silly for April Fools’. If you ever wanted to encounter the dread Bubble Dragon or a herd of Blink Mammoths, this is your DM’s issue.)