Just like concepts people also have pre-conceived notions about groups built on what they've done before. Once a group has a label in kpop it sticks for better or worse (I don't think styling is very relevant).
Even if a group improves to a decent level labels stick and people will unconsciously (or consciously) look for things to reaffirm that opinion. They will always give certain groups the benefit of the doubt depending on what is being judged.
That's not to say individual ability/skills aren't important, it's one of the most important and a key reason Mamamoo excel in this department. But I don't think using Mamamoo as an example is strong evidence that concept does not matter, their most popular live stage is MAMA 2019 for a reason.