Do not make mischief on the earth refers to acts of disbelief and disobedience.This is because peace on earth is ensured through obedience to Allah, and to go against that is making mischief.
Hypocrites commit mischief by giving aid in those who are against Allah, and to support those who deny Him. Being an open disbeliever is better than being a hypocrite because, the outward appearance of a hypocrite confuses the believer and is considered an act of mischief. In surah 4, ayah 145 Allah says "Verily the hypocrites will be in the lowest depth of the Fire, no helper will you find for them"
And when it is said to them "Do not make mischief" they reply "We are only peacemakers" which implies that they claim to be friends with Muslims and the People of the Book. But as it says in Ayah 12 "Unquestionably, it is they who are the corrupter, but they perceive it not" This means that by doing so, they are not helping anyone, in fact they are corrupting others, because of their false appearances vs. true intentions. And instead of being reformers their activities produced MORE disorder in the land. And they are not aware of this, because it was an "undercover" act and not one that is outright bad (ex. stealing, murder etc.) Maariful Quran tafsir also suggests that the reason for corruption in today's world is that of hypocrisy. He states that it is obvious that robbers, and thieves are "bad" and they are openly punished by law, such that they are put in prisons, fined etc. HOWEVER, now this is very interesting For, the mischief of robbers and beasts can be controlled by the physical power of law and government. But laws are made and enforced by men. What happens to laws, when man has ceased to be man, can easily be witnessed all around us in the world of today. This implies that those legislative that "appear to be good" may not be. And that causes even more disorder and corruption to our land, than the obvious ones.