It's yet another nice reference to Accel World, and given the amount of time he needed to learn it, it's being fair to its origins.
It took kirito like one day to start using it effectively to not only beat the strongest swordsman but also perform magic that brings plants back to life. It was a dumb thing in accel world too. If you are going to use a power system I want it to be a defined power systems so I can correctly gauge peoples strength and try to figure out their actions. Imagination power sets everyones strength == ??? so anyone can do anything at any moment "plausibly":. A random 4 year old on the street could walk up and kick kirito's ass as long he really believes he can.
It's not like they are imagining something complex where intellect or creativity even come into the eqaution. It's literally just imagining "I'm gonna hit him so hard he won't be able to withstand it". Ignore reality in order to succeed. Considering the entire rest of the show is crafting a reality just to be ignored, I don't like that.
From what I got, it took him at least the whole year in the academy to get it, if not the two years ha was stuck there, and he probably only got so far in that match due to the boost of confidence of having a sword that allowed the use of 4 hit skills - and that wasn't a victory btw.
The whole incarnation system isn't as simple as imagining you can do something. You have to blindly believe that what you want to do is possible. You basically have to overwrite your own view of the world in order to overwrite the system. Took Haru a whole week of spartan training to barely learn a single technique, and all that time for Kirito to notice that something like that was in play and grasp the basics of its workings. It's not exactly an ass pull like some make it out to be.