There are quite a few lines inside the car. I think that the High Voltage ones are all RED so as this one is yellow, I doubt that you need to worry from a safety point of view. From a customer service point of view, NO it shouldn't be exposed like that. I have looked under both of my seats and I don't have any yellow wires that are sticking out. There are wires there for the heated front seats, and airbags, but they are well under the seating area and not something that you would notice if sitting in the back seat.
Your description and photo doesn't really explain how far under the seat this yellow wire is. If you pull the seat fully forward, is the wire exposed and in plane sight? From the photo, it also looks like some of the carpet around the wire is cut away. This in itself is not an issue if this is under the seat as they run wires to the chairs and a hole is the only way to get the wires out. This is not something that is a wear space or will likely get enlarged unless you have someone on their knees pulling at it.
You have three options.
1. Complain to the dealer when you go in next and say that you don't think the work was done well enough and ask if they can better clean up the wires under the chair that might have been disturbed when they did the work last. (it's not a requirement that you have to inspect their work right after it is done.. you are assumed the right to expect it done correctly).
2. Fix it yourself. If you have some good duct tape. (i suggest the black gorilla gaffer type tape) you can cut a small piece 3"x3" and slip part of it under one side of the hole and stick pack the carpet together that will hide the wire. Likely that is what a dealer will do if you take option #1.
3. Simply ignore it. But a set of floor mats for the front and back. (i would be surprised why people don't do this if they are planning on keeping the Niro for a while). I got the Tuxmats that have a bit of a lip for the rear passenger covers and it will stop you from seeing what is going on under the chairs in front of you. You really shouldn't be looking at them anyways like they are wires running under the dash.
2018 Niro EX - Ocean Blue
GTA - Canada.