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The entire concept of dunsparce was being a troll, a jab at the player thinking this mysterious final mon in the dex would be good, just for him to be average.
Dudunsparce expands more onto this, making him now a troll at the community thinking they would get a cool dragon-like salamander for his evolution, and even doing the "extra rare" thing again with the four-segment form which does absolutely nothing.
This might be a lazy design on it's own but with context, it's just great.