I do not know what you mean about "how wrong they were about Pluto’s appearance".

Since when I was very young and until now the amount of information about Pluto has continuously increased, so now we know much more about it.

For example now we know that Pluto is practically a double planet, having a relatively very large satellite. This was not known when I was a child, e.g. at the time of the first NASA Moon missions.

However, I do not remember anything wrong. Many things that have been learned recently were previously unknown, not wrong.

If you refer to the fact that Pluto was reclassified as a dwarf planet, that is also a case of information previously unknown, not wrong.

This planetary reclassification was not the first.

When Ceres was discovered in 1801, it was considered the 7th planet, after the 5 planets known in antiquity and Uranus that was discovered a few years earlier. (The chemical elements uranium and cerium, which were discovered soon after the planets, were named so after the new planets, as their discovery impressed a lot the people of those times.)

However, soon after Ceres a great number of other bodies were discovered in the same region and it was understood that Ceres is not a single planet, but a member of the asteroid belt.

Exactly the same thing happened with Pluto, but because of its distance, more years have passed until a great number of bodies have been discovered beyond Neptune and it became understood that Pluto is just one of them, i.e. a member of the Kuiper belt, so it was reclassified, exactly like Ceres.