Everyone, this let's be nice to the hero party this time since, in this manga, the party leader is a good person and the other two members just gave a white vote about his removal, it's the monk who wanted to become more popular in the rankings by adding a more handsome/beautiful/cute man/woman who can use flashy support buffs because debuffs are not visible in plain and aren't flashy, so let's be only evil to the monk who was an idiot
Though, since we're looking from an outside perspective and we have knowledge from RPG games, we know how good or bad (depending on who receives it) a debuff can be, but to those in this manga universe, it wasn't mentioned any type of monster who gives debuffs, so it's reasonable to say that people wouldn't know how good or effective a debuff is than to when they receive a buff since they know how much it strengthens and help them
And just a little more to add, maybe the other members didn't realize the great amount of help the MC gave either by
a) they never fought against stronger enemies, so they only fought against high tier enemies after joining the party
b)they may think it's because of the MC's friend and that they got stronger (or at least more competent)
c)Both options