I want to use this in my prose in a context as follows - "The
condition of roads was "worse than mediocre" - what can be used here?
The word you are looking for is poor.
You can't do the job with only one word. You need to add at least a clause, for example:
"the condition of the roads was poor because they had not been paved, but only patched, for the past 15 years."
I write as a connoisseur of roads on the poorish end of the spectrum, from the fair road I live on (winding, narrow, in need of repaving, no shoulders and charming) to some dreadful 4WD roads in Colorado (barely passable by a large SUV with high clearance.)
Mediocre is not a word used to describe roads. (A performance can be mediocre.) For roads, in the sequence quoted by user Lawrence, skip mediocre and go from fair to poor. (If you are in New Hampshire, "middlin'" can be used for anything.)
Abysmal, Awful, Bad, Poor, Mediocre, Fair, Good, Great, Excellent,
A word about the upper end of the sequence. A road can be good or excellent, but great sounds odd, applied to a road. As for amazing or phenomenal applied to a road -- you'd sound like a hick seeing a highway for the first time. (Great, amazing or phenomenal can be used to describe a performance.)
I'm not going to get far into the opinion-based argument, except to say that my fair road might be a good road by the standards of someone who lives several miles up a dirt road. But most people will know pretty much what you mean by a poor road -- sort of a 15 mph road.
As for passable or impassable, applied to a road: it depends on what you are driving and how well you drive.