Nobody can be compared to Ali for any reason.
He transcended sports. Easily the most famous athlete alive. Maybe Michael Jordan has surpassed him recently, but the time of Ali's retirement, he was a hero from the suburbs of the Midwest to the slums of Botswana.
My next door neighbor doesn't even know who Fedor is.
Fedor can not be compared to Ali.
Ali was the Heavyweight champion at a time where there was one belt and one champion, not debate over who is the best from UFC or Pride. There was the heavyweight champ of the world and to be the best you had to beat the best.
He over come adversity, stripped of his crown and exiled from the sport. He came back and regained his crown.
He thought with and beat the best there ever was (arguably) Foreman, Liston, Frazier etc
As you say, he was the most famous man in the world and was voted the greatest sports man on the 20th century to this day everyone knows who Ali is yet he hasn't boxed for decades....only mma fans know of Fedor (even some casual fans only know the UFC stars)
I know it's not his fault that the populairty of MMA at his time resulted in his lesser profile than a top boxer and also he can't be blamed for the talent pool being split over different organisations BUT facts are facts and Fedor spent too long not fighting top guys.
During his days in Pride he fought and beat pretty much everyone avaliable for him to fight. He is a legend, but for circumstances beyond his own control he never will compare to Ali...even relatively speaking in MMA terms in my opinion (he just doesn't have the same calibre of fights or dramatic fights on his record for that, he should have joined the UFC years ago)