Having a simple mind isn't always a bad thing; a person with a simple mind can effectively resist spiritual attacks.
Lu Yang was very doubtful whether his spiritual attacks could harm the Man Gu endowed by his ancestors.
Ordinary people counter spiritual attacks head-on, seeing which side has greater spiritual power and who uses it better.
But it's different for those with simple minds. They are like cotton, able to cushion the impact of spiritual assaults.
Based on descriptions, the Nascent Soul of Man Gu is formidable, but when applied in practice, it has a significant flaw—it can't fully recognize all ancestors.
One can't blame Man Gu for this; considering the cultural level of the Ancient Barbarian Tribe, very few portraits have been handed down. Those that were came from occasional passing cultivators skilled in painting.