The chart's smart labels strategy will try to fit the labels in the available area without overlapping. However, if there is very big amount of labels, the labels may still overlap one another. In order to minimize or resolve this you can provide more space for the labels. An option which you can try is to set the RadiusFactor property of the PieSeries.
Note that the RadiusFactor works in relative units between 0 and 1.
I hope this helps.
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