Why is Wynaut 'The Bright Pokemon'?

Question

That's it. Why is it called the bright Pokemon

Answer 1

Answering because I have further detail to add that helps elaborate.

Wynaut's type as the Bright Pokémon translates to the Merry Pokémon in Japanese.
Source: https://bulbapedia.bulbagarden.net/wiki/Pokémon_category

Now, to find out what makes Wynaut the Merry Pokémon, let's look at some of its Pokédex entries. If we look at its Pokédex entry in Ruby, which was the game it was introduced in, it says

Wynaut can always be seen with a big, happy smile on its face. Look at its tail to determine if it is angry. When angered, this Pokémon will be slapping the ground with its tail.

Focus on the first sentence here and that shows why it is the Merry Pokémon, it's always smiling.

Source: https://m.bulbapedia.bulbagarden.net/wiki/Wynaut

Answer 2

Most likely due to a mistranslation. In Japanese it is referred to as the "Merry Pokemon."

Source: https://bulbapedia.bulbagarden.net/wiki/Pokémon_category