It would be nice to add rain and mist on Nauvis, similar to Gleba. It's so far the only planet which hasn't weather.

Done in 1.6.0!
Nauvis can now have rain and lowland fog over its lakes like Gleba, plus falling leaves, pollen, golden sunsets and wind gusts. It's all off by default, since vanilla keeps Nauvis clear, so turn on what you like in the startup settings.

Thanks a lot! You should add the same weather to Ligumis if it's not already done.

Lignumis already has it: rain with fog and rain sound, on by default (startup setting "Lignumis: enable rain").

I wondered if it also worked with alien biomes?

Mostly yes: rain, fog over water, golden hours and wind gusts work as usual. Falling leaves and pollen won't show up on a fresh Alien Biomes map, though, since they're tied to vanilla trees and grass, which Alien Biomes replaces with its own.