I’m wondering recently if the spirits of the dead can still perform magick, and if they can perform evocation like we do.
What i mean is what if in the afterlife we won’t be able to evoke spirits and we’ll have more difficulty to communicate with them
Shades aren’t people, they’re echoes, so they would have no need for evocation or magick.
There is no such thing as the “afterlife.” That is a fantasy concocted by people afraid of dying. The soul moves on to its next incarnation or, if advanced enough, ascends and either merges back with Source or creates its own realm to exist in. Some ascended choose to come back to earth but there seem to be specific rules that govern what they are allowed to do here.
wow that’s a sad perspective. i dont wanna reincarnate 
Then level up and escape the reincarnation trap.
They still do magick, just not evocation… By the time they were alive they accumulated their own magical powers, many strong witches can become Guardians of Nature/Places they can also becomes part of a Legion in the Espiritual World if they want. Some of the Legions they can be part is the Exu Legion of Kimbanda, where they still mantain part of their personality but they accept to mantain a pattern in their appearance (possession/“incorporação”). Many traditions talk about becoming Ancestors and living in the City of Ancestors/City of The Dead, I wouldn’t call them shades, but they’re not human in the full meaning. They still need to eat and they still have their personality/likes and deslikes.
Some humans, that died and didn’t have a sufficient strong spiritual body can dissolve and become husks or becoming a hungry ghost (what here in my country call Obsessores). That happens usually with people that died while in extreme addiction, or suffered a very violent and traumatizing death, then the ghost can “become heavy” and go to a part of the Spiritual World (where the Buddhism will call the “Preta Realm” and here in my country they call it “Umbral”) and they can attack and vampirize the living.
I would like to help people in the afterlife, specially people with mental health problems like me
So start working on getting more alliances with angels and chtonic deities that can help you, and is also important to work on your own energetic system/spiritual body (via trainings like Qigong or other practices).