Translation:In the city you will find more houses than trees.
Yes it is. “Pli” is an adverb and cannot be used to modify a noun (domoj, arboj) directly without a preposition.
If you derive an adjective (“pliajn”) you change the meaning; “plia” means “further,” “another.”
I see no way to express the given meaning without “da.” If you disagree please give a complete sentence.
I wonder if it is really true that there are more houses in cities than trees. Many city seem to be mostly apartments for residences, not houses, and they have trees on the roads, and often many small parks and a few big parks with lots of trees. This likely varies a great deal depending on where the city is.