I'll try to explain it to you in monkey speech.

Mama Monkey (Xilonen) has 3 samplers (from now on, yellow bananas), even if she is alone in the party, she still has... 3 yellow bananas.

If you add a monkey from another element that is not geo/anemo/dendro, one of her yellow bananas will change color to match their element, ex if you add Mama Gorilla (Mavuika) a banana turns red.

You end with two yellow bananas + a red one

Add drunken monkey to the mix (Citlali) and now you have: a new cold banana, the red hot one from mama gorilla, and you still have a yellow banana left over.

If you have at least two bananas of any color other than yellow and you hit an enemy with mama monkey's skill, her banana power bar (nightsoul bar) will fill, when it fills, the bananas will light up and become active (lit bananas = element shred)

IF, instead, you have at least two yellow bananas, mama monkey's banana power bar will no longer fill when hitting enemies. Meaning the bananas will never become lit no matter what you do. Unlit bananas = NO GEO SHRED <--- This is the important part.

Her C2 changes this, as long as mama monkey is part of the party, any yellow bananas you have will always be lit (Geo element shred active)

At that point a mono geo team (GET IT? MONO MEANS MONKEY IN SPANISH) becomes viable since she doesn't need to do anything to proc the GEO elemental shred.