I don't think the love of HtTYD is a Millenial thing so much as an internet thing. With the exception of Pinterest, everything on the internet skews toward nerdy males; they are disproportionately represented on any website, even Disney fan boards.
Hence, the online love of Wall-E (nerdy loner, ends up with out-of-his league dream girl), but hatred of Cars (arrogant jock wins everyone's love and respect, ends up with in-his-league dream girl)--in complete contrast to box office performance and merchandise sales in the real world.
Or the irrational hatred fanbois (and I do mean boys, especially on Twitter) of a nerdy bespectacled wizard have for a temporary DHS event featuring a blond teenage queen who is exactly the kind of girl who never would have talked to them in high school.
HtTYD was a decently received cartoon that really spoke to nerds; they can't seem to come to grips with the fact everyone else liked it fine then forgot it existed 10 minutes after leaving the theater.