Often used with nouns to describe something as quite average, not particularly outstanding. The hotel had middling facilities.
Pro Tip 2/3
Connotatively Neutral
Doesn't strongly sway towards positive or negative, but implies 'just enough' or 'sufficient'. His performance was middling, neither extraordinary nor poor.
Pro Tip 3/3
Not for Extremes
Avoid using 'middling' when describing something very high or low in quality or size. You wouldn't say a mansion has a middling size.