Why Does “A Bit Ordinary” Mean “Not Very Good”?

In everyday Australian conversation, a bit ordinary is often not a neutral way to say that something is commonplace. It can be a softened negative judgement meaning not very good or disappointing. Oxford Learner's Dictionaries likewise records a disapproving sense of ordinary for something lacking notable or interesting qualities.

A: How was the motel?
B: The room was clean, but breakfast was a bit ordinary.

B is saying that the breakfast was disappointing. A bit softens the criticism, although tone can make it sharper. Ordinary still has its neutral sense of usual or unexceptional, as in an ordinary day. Whether it carries criticism depends on what it describes and how it is said. In formal feedback, disappointing or not up to standard is clearer.

https://www.oxfordlearnersdictionaries.com/definition/english/ordinary