The Buddies won at Celtic Park for the first time since 1990 to leave Celtic still 23 points behind Rangers, which closing in on its first top-flight title in 10 years.
"It's a low point in terms of the performance and the result," Lennon, who has clung on to his position for months despite the desire among Celtic fans for him to be sacked, said.
"There is a lack of belief and lack of quality – it's not just individuals. It's a collective, and it's symptomatic of the season."
The club's long-serving chief executive, Peter Lawwell, announced on Saturday (AEDT) he will be stepping down at the end of the season, but the pressure is building on him to make a change and salvage second in the league and a place in next season's UEFA Champions League qualifiers.
Kristian Dennis fired in the opener for St Mirren in the 18th minute before Odsonne Edouard levelled just after the half-hour mark.
However, Ilkay Durmus restored Saints' lead in the 37th minute, with more questions around centre-back Shane Duffy's defending as the Ireland international made another glaring error.
Celtic has now won just one of its past six league games.
Aberdeen is now five points adrift of the Hoops after being held 0-0 by in-form Livingston.
Hibernian moved to within a point of the Dons in fourth with a comfortable 2-0 win at Dundee United thanks to goals either side of half-time by Darren McGregor and Australia's Martin Boyle.
St Johnstone came from 2-0 down to beat Kilmarnock 3-2 at Rugby Park.