Yes, it is possible to get to Plitvice by Uber, however, there is no Uber service from Plitvice to elsewhere.
I just checked some prices and info on my Uber application and I got the following:
Bear in mind that these prices are changing all the time; right now I am checking them for April and my guess is that in July and August they may rise, but you never know, I may be wrong.
Check for yourself closer to the time of your departure.
In my opinion, getting Uber to take you to the Lakes may be a good idea if several friends are travelling together, let’s say four of you, so Uber ride can be cheaper and much more convenient then taking (and waiting) for local buses.