I wouldn't say adding acidity makes her recipe better. It comes down to assessing the task vs assessing the person. In a school, you're really supposed to assess the task. However, schools also need to set clear marking criteria so students know exactly what is expected of them. It doesn't happen here, which is the source of angst and confusion.

Souma's past creations are based on the idea that "I have to serve the customer something". Shinomiya runs his place thinking "You give the customer exactly what is prescribed. Nothing different." As perfectly put by Ryll, the problem was his dooming a number of students by default regardless of skill.