I understand that you are simulating a DC-DC buck converter using the Simscape Power Systems. There is some oscillation at the beginning of simulation.
After going through your attached model, I think there are two reasons that cause this oscillation:
1. In the buck converter circuit, you have modeled resistances in addition to the ideal components, such as in the "Ideal Switch" block and in series with the "Inductor" block and the "Capacitor" block. Even with small values, these resistance values will lead to nonlinear behaviors of the converter. As a result, you may not see a nice rising curve at the beginning of simulation.
2. In the controller that you designed, the poles and zeros are all with very large absolute values. This means that the dynamics associated with the controller are fast. The fast controller dynamics may also give rise to this oscillation.
Besides these two reasons, I also noticed that the total simulation time is only 0.1 ms. This is not usually the time range used in Simscape Power Systems simulation. If you look at the simulation over a longer period of time, the oscillation is less noticeable.
Based on the power level you are simulating, I also recommend you to use the Simscape Electronics to build the model. The Simscape Power Systems are usually used to build models for electrical power system modeling at a higher power level.