I think for a first attempt it's okay, but there is quite a bit you could do to improve the scape. With a little bit of thoughtfulness on how you are placing the rock things really come together. Lets go into some basic nature style aquarium aquascaping principles:
In a small aquarium like this you really want to create one focal point. Focal points can be created by positive or negative space. The focal point should draw the viewer to naturally rest their eyes on one spot; this allows the viewer to relax and contemplate. Aquascapes with too many or too few focal points are stressful for the viewer.
Here is an Iwagumi scape that creates a focal point with negative space:
An Iwagumi scape that creates a focal point with positive space:
The Golden Ratio
The Golden Ratio seems complicated at first, but it's really not. The basic idea is that there is a ratio of visible space that is most appealing to the human eye. In photography it's taught as the principle of thirds. In aquascaping there is really only one number you need to understand: 1.6. The Golden Ratio indicates that 1:1.6 is most visibly appealing to the human eye. What does this mean for aquascaping? That your focal point should not be centered in the tank. To figure out where the focal point should be in your tank, measure it lengthwise and divide by 2.618. This will tell you how many inches or cm the focal should be in from the left or ride side of the tank.
A quick example (Standard 10 gallon tank is 20" wide): 20 / 2.618 = 7.64".
So, you can take a ruler, mark the two points on your tank that are 7.64 inches from the left and ride side and choose one of those points as the focal point.
It seems like you already understand what Iwagumi is but there are a few tips that you can use to really improve the layout. One, always use an odd number of "main" stones. This prevents you from laying them out in an "even split" and inadvertently violating the golden ratio. Second, you should choose a direction of water flow in the tank and the main stone should "flow" in that direction. Imagine that the flow of the tank has eroded the main stone over hundreds of years - the entire idea behind nature style aquariums is to try and create a scene from nature. The secondary stone can oppose the flow of the first stone to create a dramatic affect or all the stones can flow together to create a tranquil, peaceful, aquascape - it's really up to you. Regardless, just keep trying different layouts until you find something you like.
Finally, if you're interested, this is a very traditional style of Iwagumi layout called Sanzon Iwagumi. Each "named" stone has a very specific purpose. You can read more about it here
[The Green Machine].