The Blues, who had Cesc Fabregas sent off, saw their 16-match unbeaten run ended by two goals from Saido Berahino and one from Chris Brunt.
Mourinho, whose side won the title with three matches to spare, acknowledged he and his players were “guilty”.
But he added: “I think the top contenders are also guilty because they let us win the title so early.”
West Brom went in front through Berahino’s 20-yard curler before midfielder Fabregas was sent off for kicking the ball at Brunt’s head from 20 yards away.
Berahino added a second after the break and Brunt completed the scoring to inflict just a third league defeat of the season on the champions.
Mourinho said: “I can’t just blame the players, I have to blame myself. I also lost that salt and pepper you need during the week to work at a high level.
“We are champions. You can’t expect us to go on the bus crying because we lost 3-0. Am I happy because we lost? Of course I’m not happy. Am I sad? No. The ideal scenario would be to be champions, close the shop and go home.”
Fabregas’s red card was his second in the Premier League – his first coming for Arsenal back in 2006 – and Mourinho said it was “harsh”.
Referee Mike Jones was dealing with a spat between Chelsea striker Diego Costa and Baggies defender Gareth McAuley, with several other players crowded around them.
Fabregas, standing out on the right wing, kicked the ball back towards the players, hitting Brunt on the head. West Brom captain Darren Fletcher responded by squaring up to the Spaniard.
“Where is the danger of the situation, where is the aggression in the situation to get the red card in a friendly game?” Mourinho said. “I really don’t understand.
“I think Fletcher was aggressive, he pushed him in the chest because he is experienced and knows where he can push for a yellow card. He is much more aggressive than Fabregas.
“For me, a top referee, a stable, big personality and in control of the game, goes there, two or three words and it’s done. It’s a bizarre red card.
Mourinho and his West Brom counterpart Tony Pulis embraced warmly in the tunnel before the match and stood next to each other, arms across each other’s shoulders, for the final few minutes. — BBC
“I’ve managed against him twice now and have managed to beat him twice – once at Crystal Palace and now once at West Brom,” Pulis said.
“He’s been very, very gracious in defeat. For all we’ve done tonight, they’ve been worthy champions, a fantastic football club and they have some wonderful players.
“I’ve seen some people criticise them and it’s just unbelievable. They’ve been, by a country mile, the best team in England this year.” — BBC