There are several recurrent symbols in the Kraina Grzybow series and media.
A very popular family activity in Poland is Mushroom hunting. There are huge dark forests in Poland and this allows mushrooms to grow. There are also apps that can identify the different types of mushrooms. Agatha could have gotten lost in the forest, and this could be why Agatha’s mother is always looking for her. This also could be why Malgosia the squirrel is a squirrel, because squirrels are plentiful in forests, and why there are some visuals of insects, and in PU #4 there is a camera view of trees going around in circles. So, Mushroom land could not be about Magic mushrooms at all.
Apples in the series have a connotation as mushrooms. This began in the first episode where Maggie tells Agatha that the "Apple" she's been "Eating" is actually a mushroom. This gives rise to the theory that Agatha is on drugs, specifically Psilocybin Mushrooms, which are known to cause hallucination. When the apple is shown, it always has that connotation. If you were to replace "How to Effectively Apple" with "How to Effectively Shrooms," it would make far more sense. When Agatha is trying to eat the Apple, she doesn't just eat it. She explains there is "Upper Apple (The skin)," "Mushroomland (The part with the drugs)," and the "Paper (Tasteless inner part)", then proceeds to put it in her mouth, chewing it distastefully. She hallucinates wildly, then as it begins to die down, she thinks again, but feels depressed until Maggie (?) cheers her up, and shows her that she was high the whole time and that that "Apple" was actually a 'shroom the whole time. Later on, during "The Apple Escape," Agatha has to wander streets in a kind of "Withdrawal" state, depressed and tired, toking for enough wild "Apples," just growing on the ground LIKE MUSHROOMS to give her the high she needs, all while not being caught by Maggie-3000s (Ed note: I think they're police). The Facebook post on December 23, 2016 shows her in a high again on a vacation with Maggie, but not as strong a high as before, as she notes she is still not happy as before. We will have to wait and see.
The symbolism of cow could possibly be because “magic mushrooms” grow in cow poop.
Prisms make an appearance in Smile Guide: The Apple Escape. The significance of this symbol is yet to be revealed, but prisms can be found all over Mushroomland.