The habit of sharing posts is something not only built into the system, but encouraged further by badges for giving out special kinds of links. We do want people to increase the visibility of their posts, because that increases the visibility to those who actually need the valuable information contained in those posts.
The only questionable part here is the framing. Which, not unlike asking for spare change from a passerby on the street, is simply asking for those within range to not just look at the question, but vote for it as well. This particular case might be in the context of an event, but we can even divorce the event from the situation since the hat in question is derived from the same requirements as a badge. It's just as realistic outside of the context of this event as it would be if it happened at any other time in the year, and the framing's effect will be the same.
This kind of behavior is thus identical to pestering people in comments to accept your own answer, or to upvote while accepting, or any other large number of behaviors all related to asking for votes. In fact, it's really just the same as posting a link to a post for sharing, and then commenting "Please upvote me!". Just as one might flag such noisy commentary for removal, the same can be said for this situation - community agreement that the comment is undesirable is required for the flag to take effect. That the sharing link is conjoined to the commentary, well, that is a consequence a user should've considered initially.