Ijma is a mode of transmission of knowledge, not the interpretation of the knowledge. For example, Ijma of Muslims has transferred Quran from one generation to another, here we only rely on that Ijma to take those words as Quran from them, not their interpretation. Interpretation of those words will be subject to the same principles of linguistics as we apply in understanding any other text. Tawatur also represents transfer of perpetual adherence to religious actions from one generation to another with a non-breakable chain from its origin till the current time.
Herd mentality has to do more with thought process and behaviour of an individual influenced to conform with others in the group.
There seems no apparent comparison between these two terms.
Please also go through the following videos of Ghamidi Sahab to understand the concept of Ijma, Tawatur and Established History:
Discussion 55834 • Reply 55883
Discussion 55834 • Reply 55884