"Es la peor zona."
Translation:It is the worst zone.
Lo and la are articles in this case - neutral and feminine, respectively. You could also say "el peor" to make it complete.
While using el and la in front of comparative adjectives makes it refer to an actual noun (Esta area es la peor - This area is the worst), you use the neutral lo if there is no noun but you want to describe the principle of the adjective itself: Quiero lo mejor para ti - I want the best for you. (Not the best life or the best strawberry yoghurt, but just "the best".)
"It is the poorer area" no good? Duolingo: peor = poorer, zona = area; where did I err -and lose a heart -with "It is the poorer area"? (I was thinking this "unit" was talking a lot about places so thought it may be describing some of the different places a visitor might see, such as "the poorer area".)
Two reasons. The more important one is that superlatives (the best, the brightest, the worst...) always appear in front of the noun.
And then peor, which is the comparative form of malo, is an indication of quality, or "goodness", something deeply subjective. You'll mostly find such adjectives in front of the noun.