I have seen lot of people when they are as guests and are served food at the house of host, and if that food tastes bad, some of the men talk rude/bad things behind, about the food. What is the Sunnah regarding this? Thank You
Abu Huraira reported: The Messenger of Allah (SAW) would never complain about food. If he desired something, he would eat it. If he disliked it, he would leave it.
Source: Ṣaḥīḥ al-Bukhārī 3370, Ṣaḥīḥ Muslim 2064
If the host really presses you for an opinion, try not to hurt their feelings while answering. There is also another Hadith about allowing a husband to lie about her cooking & praise her inorder not to cause her to feel bad.
EDIT: W.r.t below comment by @Kilise:
Asma bint Yazid narrated that the Messenger of Allah said: "it is not lawful to lie except in three cases: Something the man tells his wife to please her, to lie during war, and to lie in order to bring peace between the people."
Reference : Jami` at-Tirmidhi (Vol. 4, Book 1, Hadith 193)