I have often seen the term used with a negative connotation. According to OLD plethora is a synonym of excess and means:
- an amount that is greater than is needed or can be used.
Also its etymology suggests a negative connotation:,
Plethora:
- 1540s, a medical word for "excess of body fluid," from Late Latin plethora, from Greek plethore "fullness," from plethein "be full" (see pleio-). Figurative meaning "too-muchness, overfullness in any respect" is first recorded 1700.
(Etimonlyne)
According to the Grammarist, however, the term does not necessarily imply a negative connotation:
Plethora:
- is an overabundance. The excess here is key; in traditional usage of the word, a plethora is too much of something. Today, however, the word is often used as a synonym of plenty or many, which imply abundance but not necessarily overabundance.
Examples
But decision science has shown that people faced with a plethora of choices are apt to make no decision at all. [Daily Beast]
The plethora of mixed motives for the west’s engagement with the Arab world make doing the right thing harder in the Middle East and North Africa. [The Guardian]
I spent a few hours Monday on the ministry’s website, and was nearly driven mad by the plethora of acronyms. [Globe and Mail]