Just like English has a set phrase "more or less", Polish has a set phrase "mniej więcej". It would be very strange (I believe) to say "less or more", and even stranger to say "więcej mniej". And Polish doesn't use anything for 'or' here.
Shouldn't "less or more" be accepted. Just because "more or less" is an idiom in English really throws off learning how it should be"mniej więcej" in Polish.