I think it would have been fine if it was after a Diablo 4 announcement. The biggest problem was that they tried to push a mobile game as the big reveal of 2018 Blizzcon to a room full of PC/Console gamers.
Add in the fact that it had been 6 years since Diablo 3 and people were anticipating a new Diablo game, especially since rumours were flying about Diablo 4 being in development(and most of us figured it was coming sooner or later).
That said all the bad rep will mean nothing anyway. Regardless of if it actually affects how well it does in North America and Europe, Immortal will do really well in the market they likely primarily intended it for in the first place: China.