Spurs looked jaded and uninspired for much of the game with Dominic Calvert-Lewin's second half header condemning them to a 1-0 loss.
After the match, Mourinho revealed his disappointment at the lacklustre display.
"Against good teams like Everton if you let them play from the back they are comfortable with very good technical players. If you don’t press them you give them ammunition," he said (via Reuters).
"We were lazy in our pressing. That is a consequence of bad fitness, bad pre-season. Some players didn’t even have a pre-season. Some players had a wrong state of mind. I don’t want to speak about it - I’m not sure about it."
Mourinho also criticised referee Martin Atkinson. The former Chelsea boss was unhappy that Lucas Digne was allowed to inch the ball forward, before whipping in a free kick that led to the game's only goal.
"I didn't like that so many referees on the pitch, on the touchline and in front of the screen to allow a free-kick to be taken five or six metres in front which create a totally different angle of cross and totally different angle to a cross to attack the ball and put the defensive zone line," he added.
"I can complain now as I was complaining before the goal. I was complaining already with the referees before the goal."
Tottenham have little time to dwell on Sunday's poor result with a Europa League tie against Lokomotiv Plovdiv to come on Thursday. With the games coming thick and fast, Spurs have strengthened over the summer.
New recruits Matt Doherty and Pierre-Hojbjerg both endured underwhelming debuts against Everton and Mourinho remained tight lipped when quizzed about the club's interest in Trabzonspor striker Alexander Sorloth.