I wan't to express, that I disliked Rick and Morty season 3

[Der/die AtheistIn]

Actually - that would be fantasy. Sci-fi generally builds on our existing reality, using technology and theoretical scientific and philosophical concepts to explain what appears to be supernatural. The Pickle Rick episode is pretty difficult to explain in those terms. It's mostly just wacky cartoon fun. 

I have a different opinion. Sci-fi does have to do with technology and theoretical concepts to explain what happens to be impossible or unknown if possible in our real world. Fantasy either doesn't give any explanation or gives magic as an explanation. Rick - a scientist - turned himself into a pickle using science, not magic, thus making it sci-fi. A ridiculous sci-fi but sci-fi nonetheless.

Yes, for the most part. But in the show's multiverse there clearly exist exceptions to every rule. And most Ricks seem to be aware of that. If there can be mentally deficient Ricks, there can be particularly mature, or gifted Mortys - all of which is consistently depicted in the episode. You may find it unrealistic, but again - it's a cartoon.

I talked about it in the OP. I do see it as a possibility that there are universes in which children are as mature as adults or at least for  some Mortys to be as mature as adults. What bugs is that the show never established that this is the case with evil Morty. Maybe the show doesn't want to tell it directly, but I personally don't see any (good) hints.
He does act mature, however his opponents act so immature that he comes across as adult like mature only in comparison to them. This doesn't prove he thinks like a kid, it just doesn't give any hint that he doesn't. These supposedly genius Ricks are so easy to manipulate that not only does it not prove that evil Morty has the mind of an adult, but he also comes across as a not so good of a villain, not because of his own traits, but because he has such weak opponents.

Maybe. But wasn't that kind of what happened when Evil Morty originally appeared? Perhaps the creators didn't want to repeat the same scenario.

I don't see any problem if they repeated the scenario. Evil Morty wasn't caught the first time, if he pretended that he is a normal Morty nobody would suspect him. This time he could make a self destruct system on Rick so that nobody could find out he was mind controlled. The other Ricks know that evil Rick was mind controlled, what if they became scared of mind control? What if evil Morty took their fright to his advantage? They could've also make evil Morty try to mind control someone, but fail. They could've also had him pretending that he's a normal Morty while planning other things than mind control. 
Also, why would the creators worry about repetition, when they themselves repeated two times a joke in Ricklantis Mixup which they already used in Raising Gazorpazorp? Not only that, but they repeated the joke once in the  first appereance. Rule number 1 of comedy is a joke isn't funny the second time you hear it, or at least is less funny.

Here is the joke in the first episode: 

https://www.youtube.com/watch?v=dBJFSJQvAUI[/video]

First time it was funny, because you were expecting him to use actual words. Second time, not as funny.

Now, in the newer one, it takes place before Morty's speech:

https://www.youtube.com/watch?v=t2S0_JKSm1Q

The first Rick farts with no bild-up for it and the next to basically repeat the original joke only with slightly different lines.

Call it "meh" humor if you will and you have the right to do what you want, but I don't think that it truly brings us together. So we all have something in common excluding our many differences and we are all equally bored. Look, I may have criticized it, but truth is, I find it harmless.