Praise be to Allah.
The basic principle with regard to adults watching cartoons for fun is that it is permissible, so long as they are free of anything that is contrary to Islamic teaching such as corrupt beliefs, music and so on.
But that should be done on rare occasions and only for a short time.
It should be noted that Allah will question each person on the Day of Resurrection about his life and his youth, and in what ways he spent them, as it says in the hadith that was narrated by at-Tirmidhi (2416) from the Prophet (blessings and peace of Allah be upon him), who said: “The son of Adam will not move on from standing before his Lord on the Day of Resurrection until he is questioned about five things: about his life and what he did during it, about his youth and how he spent it, about his wealth and from where he earned it and on what he spent it, and what he did with the knowledge he acquired.”
Classed as hasan by Shaykh al-Albaani (may Allah have mercy on him) in Saheeh Sunan at-Tirmidhi.
With regard to the cartoon known as Pokémon, it is a Japanese cartoon series that is produced for children. It is not permissible to watch this show because it contains serious matters that are contrary to Islamic teaching, including the following:
1.It is based on the corrupt, atheist, Darwinian theory that is known as the theory of evolution, and speaks of the evolution of different kinds of creatures from lower forms to higher forms that are more powerful and intelligent. We have previously explained that this theory is false and is contrary to the teachings of Islam. Please see fatwa no. 126913.
2.The show propagates the theory of the survival of the fittest, which is another Darwinian theory. It also promotes ongoing conflict and violence, and fights that take place between the various creatures. This poses a danger to the morals and conduct of adults and children alike.
3.The show includes symbols such as the six-pointed star that are known to be strongly connected to Zionism and Freemasonry.
These are some of the things that are wrong with this show, and there are several others which you can read more about in the following link (in Arabic):
All of this makes watching this and similar shows haraam for adults and children alike.
We advise you to try to spend your time in things that will benefit you in both religious and worldly terms.
And Allah knows best.