You can use a thicker plate than that if you want (e.g. there seem to be lots of people who mount MX switches in substantially thicker acrylic plates), but it won’t be able to clip in to the other side, so the little plastic clips will get a bit bent out of their usual shape. This is fine if you are making a keyboard to keep indefinitely, but may not be ideal if you want to use the switches for something else after.
For instance, though, the typical Ergodox (such as the version MassDrop ships) uses a thick acrylic plate.
If you can get a 1.5mm metal plate, or similar, that’s better.