Learn German: Die Suppe war lauwarm und geschmacklos, dagegen war der Salat frisch. - The soup was lukewarm and tasteless, on the other hand, the salad was fresh.

The word dagegen (on the other hand / in contrast) is an adverb, not a coordinating conjunction like und or aber. When an adverb occupies the first position in a main clause, it takes up position 1 and pushes the conjugated verb into position 2. That is why we see dagegen war der Salat instead of dagegen der Salat war.

Could I use hingegen instead of dagegen in this sentence?

Yes, hingegen works perfectly here and means exactly the same thing. Both are excellent B2-level words to express a contrast. You will also often see these adverbs placed after the subject for stylistic variety, which would look like this: Der Salat dagegen war frisch.

Does geschmacklos only refer to bland food, or can it be used for things like a tasteless joke?

It works for both! Just like in English, geschmacklos can be used literally to describe food without flavor, or figuratively to describe behavior, jokes, or clothing that lack good taste or tact.

How does dagegen differ from causal conjunctions like zumal (especially since)?