The Jambos now look set for a 10th-place finish in the Clydesdale Bank Premier League after Ross Caldwell fired home a late winner for the visitors at Tynecastle.
Hibs leapfrogged their rivals into ninth spot with the victory after Leigh Griffths' free-kick had earlier cancelled out Darren Barr's opener for the hosts.
Locke told the club's official website: "The players were really flat today. We've been getting plaudits for how we've done lately, but we didn't get going. The preparation was the same.
"Hibs played at a tempo that we wanted, but couldn't match it. In a derby game, you've got to win individual battles but we lost too many.
"We probably didn't deserve to lead at half time, and we tried to stress that we needed to win second balls, then we gave away a sloppy free kick and you can't allow a player like Griffiths a free shot at goal.
"We'll look to the summer now, and try to get some new players in, and try to improve from this season."