Under the hood, those quad polygons are divided into two triangles. The cloth will not stretch well along the dividing edge, but will stretch much better along the other diagonal. That's what is causing the "sawtooth" effect.

There are also four polygons where two different "edge loops" end in a single vertex. These polygons behave differently in the cloth simulator.
Id suggest bringing the cloth back into your modeling application, and chamfer the collar edge. That'll get rid of the edge loop problem.

Once you've chamfered the collar, I'd suggest copying the edge polygons to a separate object, invert their normals, and extrude/bevel them for a small amount, let's say 0.1 cm extrude and -0.05 cm bevel.
Also extrude+bevel the original edge polygons. Weld the objects together, and now the collar will have some thickness.
Because the "back" of the collar polygons have their normals facing the other way, the dividing edges will follow the other diagonal, effectively canceling the sawtooth effect. The collar will also be stiffer, just like a real life doubly layered collar would be.
And the final bonus will be that the cloth doesn't look "paper thin", like many dynamic clothes do.

Before you do the chamfering, I'd suggest subdividing the shirt once, making the shirt polygons approximately the same size as Aiko's polygons. The cloth simulator tends to work better when the polygons of the cloth object and the collision object are roughly the same size.

And I'd suggest lowering the Collision Offset and Collision Depth to 0.1 or 0.2 both. In your simulation, it looks like you've kept them at their default values of 1.0 - which means that the cloth will try to keep a distance of 1.0 centimeters from Aiko. 0.1 would equate 1 millimeter, which is a far more realistic cloth-skin distance.
Low Collision Offset and Collision Depth values require higher poly clohting, however. Low poly clothing will very likely suffer pokethru.

Hope this helps,

Steven.

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