Is it allow to make criticism of how system work in league and here?
Absolutely. Just keep your criticism respectful and constructive, basically. Not only is it more board-appropriate, but it's more likely people will take your perspective seriously than if you're ranting or insulting.
Do you ever take criticism as chance to improve yourself and game experience for players?
Always. At least, on the former. I'm not a Riot employee, and so I have no say over the in-game experience.
Or you just shut people who disagree that system is perfect and players are the only ones who need to improve?
Nope. The system isn't perfect. Moderation makes mistakes here and there. We update rules occasionally and frequently discuss the need for additional alterations.
Do you ever think that some of your rules maybe need changes?
Potentially! We put a lot of time into the last pass we did on the rules, but undoubtedly there's room for improvement and additional clarity.
Do you ever take suggestions???
We take suggestions and put out applications for the moderation team from time-to-time, so yes! We listen to pretty much every suggestion we get as well, although we don't necessarily agree with or implement all of them.