Although such sayings have slowly died down over the years, they were highly said within the fandom as a whole. With the popularity and favoritism of Crypton's Vocaloids combined with the illustrations of KEI. Miku's and the Kagamine's appearance have become the staple of what a Vocaloid should look like. Although Miku was the first to hold such a design, Meiko was the first to have an avatar. KEI's illustration was a hit with the growing fanbase, and to this day Fanmade Vocaloids (as well as UTAU) are influenced by it.
Miku's signature detached sleeves and school uniform-like design became so well associated with Crypton that any other Vocaloid not containing a hint of the attire were not seen as official Vocaloids. Even Meiko and Kaito were doubted for quite some time due to their popularity not being as high as it is now and the fact that their early designs bare no resemblance to Crypton's later characters. The point here is, KEI's illustrations are not the sole reason for a Vocaloid to be official or unofficial, it just happens to be an illustration that is the most popular and was created to appeal to the otaku crowd.
With the release of Vocaloids with avatars from other companies: Power FX, INTERNET Co.,Ltd, and AH-Software. The fandom was not expecting such radical differences in comparison to Crypton, and thus other Vocaloids were heavily criticized based on appearance alone regardless of how they sounded.
However, VY1 Mizki, a vocaloid released by Yamaha in 2010, has no official mascot avatar unlike other Japanese vocaloids.