Marlet, who is currently on loan at Marseille, has endured a difficult two years at Fulham since joining from Lyon.
The French international is the club's record signing at £11.5 million and Al Fayed believes the fee was a waste of money.
"Living on a salary of about £1.725m a year must be hellish," fumed Al Fayed.
"Most people in this country will sympathise with you. After all, Mr Marlet, your basic weekly wage of £30,000 is what most people can hope to earn in a year.
"In return for this paltry sum he had to play almost 80 matches, during which time he achieved outstanding success by scoring 11 league goals.
"One has to wonder if the taunts at Marseille are racist or merely a reflection on his footballing skills.
"When you think that the transfer fee was £11.5m and the club also paid him more than £3m for the two years he was there, this works out at a cost to the club of more than £1.3m per league goal.
"And by the way, Mr Marlet, haven't you forgotten something? Aren't you only on loan to Marseille?
"Unless you start delivering some real value for them, a return to hell beckons."
Al Fayed finds himself locked in a court battle with former boss Jean Tigana with Marlet's transfer at the centre of the dispute.
Tigana is suing the club for £455,000 in outstanding wages and bonuses following his departure in April, while the club are suing the Frenchman for allegedly persuading the club to pay excessive transfer fees and Al Fayed feels the club's argument is backed up by the performance of Marlet.
"It is not surprising that the club has taken to the courts to try and resolve all their concerns about this transfer," added Al Fayed.
"How on earth the club was persuaded to pay all that money for a player with such an unimpressive record and so little success is undoubtedly a question well worth answering.
"For a club that, according to Mr Marlet, 'lacked ambition', it is a huge amount for Jean Tigana to have spent for so little return."