The AABB structure (repeating characters) is common in Chinese idioms to add emphasis. 浑浑 (húnhún) intensifies the feeling of being muddled and mixed-up. 噩噩 (è'è) amplifies the sense of being in a stupor or a daze. Together, they create a vivid image of a person whose mind is like a muddy pond—murky, unclear, and without direction.
In Chinese culture, which has traditionally valued diligence, ambition (especially for the collective good), and self-cultivation, the state of 浑浑噩噩 is viewed very negatively. It's seen as a waste of one's potential and a failure to live a meaningful life.
This idiom is common in both spoken and written Chinese. Its connotation is almost always negative.