hi,
S11 is reflected wave over incident wave at source and S22 means reflected wave over incident wave at drain.
usually drain output power will be higher due to your LNA amplification. if we have high reflected wave/power at drain, it can damage your device. this is the impact of having bad S11 and S22.
since you are designing LNA, your gain requirement may not be stringent or high enough. thats why you are statisfied with S21 and only asking about S11 and S22. however, you should be worried about life time of your device! get S11 and S22 lower enough and reduce the S21 to the level you want thru bias.
best of luck!