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 an Uber ride can be cheaper and much more convenient than taking (and waiting) for local buses.