As the saying goes, those who go barefoot aren't afraid of those who wear shoes.

Both being Marquis Rank experts, why is Murong Zhen so wary of Shangguan Meng, while Shangguan Meng doesn't take Murong Zhen seriously at all?

It's precisely because of this reasoning.

Apart from the surname 'Shangguan', Shangguan Meng has almost no connection with the Shangguan Family.