As of now, if you apply column series to the stacked column with line value then you are unable to customize the data label for column values. Like if you just want to show the data labels for line value and not for stacked columns, it is not possible!
Definitely agree this would be very helpful to avoid unnecessary clutter on visuals. I have numerous reports where I'd like to show data values on a single line or column in the visual without showing the values for ALL series.
Also if you try to change the position of the labels on the column you cannot then amend the position on the line. Seems you can only specify one or the other.....