Jonny Frost.. I've had a bit of trouble wrapping my mind around, it's interesting but a strange pick to adopt straight out of that novel, rather than simply create an 'inspired' character. And it would seem to indicate a greater presence of the Joker in this movie than we currently realize.
Though I guess it could also be related to a 'flashback' sequence allowing insight into the Joker. You know, when nothing quite fits, it must be a flashback. Lol.
I suppose if he were Jonny Frost it wouldn't be the first time a minor or secluded character from source material was used as some sort of supporting role in a broader arc or media portrayal.
Jonny Frost in the 'Joker' novel, in a lot of ways, was representative of the everyman transition to ambitious, devoted 'Joker follower'/thug.. he granted a rather deeply involved view of the Joker's character and appeal from this unique standpoint. And the more facets you see of the Joker, the more he gets to 'show off' through them and be known for what he really is.
They might want to play up the devotion, following, Joker 'celebrity' and legend aspect as has been suggested. With what we've seen so far, drawing that conclusion isn't a stretch.
So while it might be a smaller role for Parrack, it would be no less significant in the grand scheme of things.