Most Logical Reason: Goomy looks nothing like a Dragon.
The Dragon type is known for such ferocious monstrosities as Hydreigon, Palkia, Dragonite, and Salamence. Goomy has literally no Dragon-like qualities, being a pink-purple slug making it more akin to the Poison type. Many people do not like how Goomy has basically butchered the Dragon's reputation. Look at this equation:
Goomy = Dragon
Charizard ≠ Dragon
If Goomy can earn the title of Dragon and Charizard can't, what's the point of him? (Note that this is your typical genwunner argument). Charizard literally looks like a dragon. Goomy looks like a snail. And snail ≠ dragon.