IMO ignorance is chiefly a lack of knowledge. If you drop someone who has no knowledge of cars into a mechanics shop and tell them to fix a car, they’re going to be really incompetent and look bad compared to the mechanic. But, if they had car manuals to read and learned some mechanical engineering and watched the mechanic work and spent time tinkering with the engine, they could figure it out.

Stupidity is a failure of logic and self-reflection, like continuing to insist that obviously wrong things are true when presented with undeniable facts (“this isn’t a car at all, it’s a plane, nobody can fix planes, that’s why I’m failing!”) holding two ideas that are in opposition to each other and not acknowledging the dichotomy (“all car mechanics are really stupid but they make these cars so complicated that no reasonable person could ever figure it out”), flip-flopping back and forth between statements, blaming other people or things (“this is all Big Oil’s fault”) and generally not thinking things through.

Both might make equally terrible cars at first but ignorance is a lot more fixable and might not be the person’s fault (like if they grew up Amish and never saw a car’s engine before), while stupidity is more about refusing to learn or self-correct when given the option.