Quanto è popolare Amalka in Repubblica Ceca?

Its still pretty popular among older generetaion that grew up on it. Some new generations (teens , kids) might know it from youtube or other sociál media. I can even imagine some new parents making kids watch the older cartoons. Hard to say honestly 
Edit: I think pretty recently new book came out about Amálka but its 1 am sorry to lazy to chceck

Mildly popular. Millennials and older know it reliably, as it aired every year as an evening goodnight story on TV. Younger people would be less familiar. Cult? No way. There is cult of Krtek, Pat a Mat, not many others I would say.

In Czechia, we gave something called “Večerníček” (basically diminutive term for “evening program”; “večer”=” evening”). It’s a short program for kids before they go to bed. There are many cartoon series, usually from socialist era that are broadcast as “Večerníček”.

Amálka is one of them. She’s largely viewed mostly within this program. AFAIK, she’s fairly well known to older zoomers like me and older generations. But I’m not aware of her being popular outside that

Amálka is amazing, it’s a part of Večerníček, like the other comment said.
If you need more children’s shows for your project, definitely look into Krtek (The Little Mole). Krtek is probably the most well-known Czech animation with quite an interesting sound design. Krtek doesn’t really speak, he just uses a few Czech words like “Jejda!”.
Also, you can look into Mach a Šebestová. Everyone, even the younger generations, know the theme song.

good fairytale never gets outdated, these are good – nice drawings and animations, good voice acting, and relatable stories

it was, and probably still is, aired by Czech TV as one of bedtime stories – people recall it despite that the serie is shorter than other bedtime story series, so yes, I would call it popular