Actual radioactive contamination depends on a large part on whether it rained when the radioactive cloud passed. Austria and southeastern Germany (Bayern, parts of BW) had rain, as did the swiss shore of the Lake of Constance and the Ticino, plus small parts of the Jurs mountains. Because of that there will be huge differences in measurement results depending on the region tested. Also take note that most cards showing radioactivity incurred from Tchernobyl use a logarithmic scale.

Plus, since chemical products of Caesium can be soluble, it's not enough to consider radioactive decay only. You need to account for typical precipitation as rain over time washes it out, which obviously is a disadvantage for Ticino when compared to CH north of the Alps or Bayern/Baden-Württemberg.

What's more, with edible mushrooms there are huge differences in how much they take in, factors of 20x or even more are far from extraordinary. Unfortunately Steinpilz and Morchel aren't among those taking in little.

With that said, if you intend to use for instance Morchel or Steinpilz anyway, you may well be better off using your own finds rather than buying in a store. Because Morcheln and Steinpilz are typically imported from eastern Europe, Poland in particular, which in some parts got hit by quite a bit more than CH.