No, if your 1 year old car needs fuel injector cleaner, there's something
horribly wrong with your car.

If anything, taking it for its first long highway trip should clear up
whatever (probably negligible) amount of dirt is in the combustion
chambers or fuel injectors.  Assuming you aren't driving through a
continual dust storm or doing 100 mph through 100 degree heat all day
or something, it's probably a lot better for your car than driving in
the city (another way of saying that stop-and-go city driving is pretty
brutal).

Twenty-one city is nothing to scoff at for that sort of car.  If you've
got the four-cylinder, it's just a tad lower than the official ratings,
which I'd attribute to these two factors:  the engine was not yet
broken in during most of that period, and hardly anybody gets the EPA
city mileage anyway, because "city driving" is so much harder to define
precisely than "highway driving."

If you've got either of the V6 engines, then your observed city mileage
is actually just a bit better than the EPA ratings.

On the highway, you should see mileage in the high twenties to low
thirties, measured over the whole trip in order to average out the
effects of hills, head- and tailwinds, etc.  This can vary with how
heavily the car is loaded, how fast you drive, where your trip takes
you, etc.

Cheers,
--Joe