Leanne Mitchell's debut single did not even reach the top 40, while her album recently missed out on the top 100.
The Black Eyed Peas star told the Radio Times: 'The record company should be freakin' embarrassed. Somebody should be held accountable.
Outspoken: Will.i.am has blasted record label
bosses at Universal for the way they handled last year's The Voice
winner Leanne Mitchell
Controversial: Leanne won the 2012 show but recently her album failed to even make the top 100
Will.i.am said that it was a mistake that a record was not ready for release within days of the winner - who was coached by Sir Tom Jones - of the BBC1 show being announced.
'Do you wait a year later? Somebody should get slapped because you waited too long.
High hopes: Black Eyed Peas' will.i.am says the record label should be embarrassed and should have rushed her music out
'The show paid all this money for make-up, the lighting guy, wardrobe person, editor, director. Where are the bloody songwriters and producers? Boom! Somebody should be ready,' he said.
'If you're having performers sing other people's songs, you should have a whole bunch of songs ready in your arsenal. 'It's hopeless to release a record so long after the series finished, especially in the attention deficit disorder society that we live in today.'
He admitted that aspects of the show, which has returned for a second series with fellow coaches Sir Tom, Jessie J and Danny O'Donoghue, needed to change.
He joked about dunking contestants in the show, which lost out to Britain's Got Talent in the ratings, in a pool of gunk.
'If it was always blind auditions, you'd get bored of that, too.
'I think something has to happen at the end that is just as unique.
'As the series nears its end it needs to be better than it was at the beginning,' he told the magazine.
'Something like where you could actually dunk 'em: 'I don't like 'em,' and they're in the water. F****** bubble singing.
'Pshhooooo....An exit button and you just catapult them out. Pshhooooo.'
The star, who is enrolling on a computer science course in September, confirmed reports that he is putting together a technology reality show, but denied that he was doing it with Simon Cowell.
'It's something I'm passionate about and we're a step away from making it reality... I'm plugged into this network of kid coders, kid robot builders, and that's a pretty cool network,' he said.
Not happy: Will.i.am also speaks about this year's show and what changes have had to be made