Marcel Maciel was a bad egg. There have been other bad eggs in the Church. In hindsight, Pope JPII probably should have taken steps against him (and some of the other prominent bad eggs). However, we need to remember that
a) Saints are human, and they make mistakes. In the case of a Pope, God would protect the Church against the Pope making errors in his doctrinal teachings, but Popes can and do make mistakes or wrong judgments about other people. Often a future Pope will set this right.
b) It is highly likely that Pope JPII didn’t have all the complete information about the bad things that Fr. Marcel Maciel was doing.
c) It is also highly likely that Pope JPII was focused on the issues involving Communism and trying to bring it down, and consequently he may have had less focus on, or been distracted from addressing, some of the other problems in the Church.
d) People who hate the Church will say bad things about its great saints. Usually as 1ke said, they don’t have good sources (a good source is a scholarly book written by a fairly neutral scholar, not some anti-Catholic Youtube or web blog).
Don’t let yourself be swayed by this, and don’t waste a lot of time arguing with people whose minds are closed.