It was like that at the start of the season (the booing of Kone against Watford), died down a little when we were actually playing well for a little while, but has been back in full effect over the last couple of months.
Kone wasn't booed when he came on versus Watford.
This issue was discussed here at the time and debunked for the fallacy it was.
The boos were for the removal of the player he was replacing which was an unpopular decision....I can't recall who it was, possibly Mirallas.