Is it wrong to say ‘sourness is the taste’?

Well, Gngram completely rules out sourness is the taste in favour of sour is the taste. And yes, you could argue that sourness is a quality related to taste.

Cambridge defines it as

the quality of having a sour taste.

But Oxford Leaner's dictionary says that it is

a taste like that of a lemon or of fruit that is not ready to eat

whereas other dictionaries send you straight to "see the definition of" sour.

I personally agree that sourness is the quality of being/tasting sour, not the taste itself, so it does look like Sour is the taste that detects acidity would be a better formulation.