The only generalization I have made about White people is that they have White privilege, which is a true statement.
What you have described is prejudice, not racism. Yes, I know what the dictionary says “racism is generalizing”, but I believe the definition of racism is prejudice + power. If someone is making generalizations about your race, and judging you based off of those generalizations, that is prejudice. It would become racism when those generalizations made against your race become institutionalized, and when the system keeps you oppressed because of these generalizations.
Someone assuming you are dumb is not a good thing, but it is not racism. White people do not experience racism, they can however experience prejudice and discrimination.