Everton captain Gylfi Sigurdsson scored the only goal of the game when he beat Mendy with a 22nd-minute spot-kick.
Mendy had given away the penalty after charging into Dominic Calvert-Lewin, who was attempting to reach his own flick-on after a long ball by home goalkeeper Jordan Pickford.
Lampard defended Senegal international Mendy after the match, insisting he was not the only Chelsea player who could be blamed for the goal.
The 28-year-old has made an impressive start since signing from Rennes, conceding only three goals in eight Premier League matches.
And Lampard was keen to recognise Mendy's contribution having replaced Kepa Arrizabalaga as Chelsea number one.
Asked if it was an uncharacteristic mistake from Mendy, Lampard said: "Yes - he has been fantastic since he has been here.
"I don't think it was just clearly his mistake either.
"I think the mentality of the team to deal with the second balls in the early part of the game was not so good.
"This is a stadium and team that is well organised and they wanted to stop us playing.
"So when you give them that head start then it is very difficult and we didn't quite do enough then.
"We knew what was coming with Everton with the physicality and in the early part of the game we didn't deal with that well.
"I don’t think just he [Mendy] was culpable. He has been outstanding since he’s been here, so I'll take that one."
The Blues failed to register a single shot on target after Kurt Zouma's 28th-minute shot was saved by Pickford.
Lampard added: "I didn't think we necessarily deserved to lose the game in terms of a couple of chances [that hit the post].
"Although, if we are honest with ourselves, it wasn't the us of recently.
"We could have got something out of the game but I wouldn't say we were unlucky because it is football and I don't think we peppered their goal with great opportunities."
After their second league defeat of the season, Chelsea are away to Wolves on Tuesday and then have a six-day gap before their next top-flight contest at home to West Ham.