While the choice of Hiyori panel is a little silly, that image of Luffy was also the one photographed for his new bounty poster, which gets revealed in the volume, so it's got some reason for being there.
Saying the cover gives the impression that Luffy and Hiyroi are close is a bit ridiculous. It's like saying the volume 103 cover implied Law and Kid would use their awakened attacks against Kaido. It's not that literal, they're all on there together because they're important characters to that stage of the story.