If you look at the Netatmo devices you can see that they have plastic caps and a long plastic notch. I'd bet that the antenna can be found in such an area...
I dont think that a small thickness will help to let the radio signal pass. Think of that very thin µ-metal sheet covers on high frequency circuits e.g. in TV or Radio inputs (where you want to stop a radio transmission). They are built to not let radio waves pass and they are thin - so I dont think the thickness plays a major role. As Bulldog wrote paramagnetic blocks radio waves. And aluminum stays paramagnetic even if you reduce the thickness.
But I made the experience that relatively small areas, where the radio waves will not get blocked (e.g. plastic caps) can make the difference between no signal and "its working". Also keep in mind that nearly all antennas have a direction. So changing the orienation of the antenna might dramatically change the transmitting capabilties.
