It’s pretty simple. If your instruments sound crappy (cheap, badly set up, out of tune, dull etc….) then your mix will too (unless the LO-FI sound is what you’re looking for). It’s predictable to use the following metaphor but it’s worth it due to its accuracy: You wouldn’t ask a professional chef to make a delicious meal out of rotten ingredients. If you consider loose tuning / dull sounds / old drum skins to be a part of your creative sound then that’s fair enough - but it’s a big risk to take and this can’t be fixed easily afterwards.