Definitely seconding CiFlower as the most disliked design in recent memory! However, I feel like redesigns are always very divisive, in some ways I find vocal synths whose initial designs were poorly received to be the most interesting.
To my recollection, Tone Rion's first design caught a lot of flack. iirc she was considered a poor imitation of Miku, overly cluttered with a garish colour scheme. This may seem surprising now people look back on her first design fondly - I think this is because of a lot of the affection for her aesthetic comes from the nostalgic charm it has developed over time. She didn't have that advantage during the ancient year of 2011, while those trends were still contemporary.
Rion was infamous for being labelled the least popular Vocaloid3, with the strongest indicator of her design's poor reception obviously being the fact that her company chose to redesign her completely (although this didn't pay off either, as by that point people had grown attached, whoops).
*edit*
Other designs I remember being divisive:
Miku Formula allegations: SeeU, Lapis, Yukari, Tianyi*, Stardust*
Kagamine allegations: Anon and Kanon
Nudity: Miku Append, Lily
Artstyle: Avanna, Bruno and Clara*, Big Al* Sweet Ann, Sonika*, Miku English, Yohioloid - my lord it was basically a Spanish/English curse
*Vocaloids marked with a star were so controversial that a new design was created. Although in Tianyi's case, it's quite possible they intended to do that anyway.