Hansi Flick's Barça have won back-to-back Spanish top-flight titles and lead the league unbeaten, while Mourinho arrived this summer after Madrid went two seasons without a major trophy.
The panorama has changed slightly, after Barça suffered their first defeat of the season at Paris Saint-Germain on Tuesday, while Madrid have won their last four matches.
Madrid are third in La Liga, six points behind Barça before Sunday's meeting at Camp Nou.
"I think the people that thought a week ago Barça would thrash us will keep thinking the same thing," Mourinho told reporters on Saturday.
"I don't think just for the Paris game, or Madrid's (4-1 win against) Leipzig, they will change their way of thinking."
Mourinho highlighted that he's only been at the helm a few months, compared with Flick, who arrived at Barça in 2024.
"People look at us like a finished product, but finished products in football are difficult because they are never finished... but they look at us like we're in an advanced phase of the process," continued Mourinho.
"We've been working for three months, out of that you can take pre-season when the players were arriving (back from the World Cup), and the recent international break, then we've been together a month-and-a-half, two months...
"This is the start of a product, we're in an initial phase where the coach is learning, analysing, bit by bit defining paths, the players are learning about the coach... comparing a team in this state (to more established sides), I think, is unfair."
Mourinho said form did not matter when it comes to El Clasico.
"We've been in a good dynamic, winning games, playing well, scoring goals, but in El Clasico it doesn't matter what you did before, it has no influence," said Mourinho.
"El Clasico is unique, normally the football world stops to watch it. Hopefully it's a great game. And we win."
Mourinho said he would not change his plans to try and stop Barcelona winger Lamine Yamal.
The 19-year-old Spain international is in sublime form and hopes to win the Ballon d'Or award on Monday.
"He's a great player and great players you respect," added Mourinho.
"One thing is respecting, another thing is to change plans, structures, forgetting your own essence. That we will not do, neither for him nor any other player of great quality we face."