It confuses me, because it is objectively not true. Listing out what we know every Avatar did (which I will do again now) she doesn't even get close to how much Roku flops.

List of what the Avatars did:

Wan:

• Started the Avatar Cycle (+)

• Locked away Vaatu, stropping 10,000 years of darkness (+)

• Closed the Spirit Portals (+/-)

• Couldn't stop the wars (-)

Szeto:

• Made the fire nation more functional (+)

• Ignored the rest of the world, leading to circumstances that would make the Platinum Affair worse (-)

Yangchen:

• Made a treaty with General Old Iron, saving the people of Tienhaishi (+)

• Relocated all the people from Tienhaishi (+)

• Stopped Bin Er from getting destroyed (+)

• Stopped Chaisee from blowing up the heads of the Water Tribe, Earth Kingdom, and Fire Nation (+)

• Ended the Platinum Affair (+)

• Made terrible deals with spirits that led to many of them becoming darkened leading to Kuruk's death (-)

Kuruk:

• Killed dark spirits, doing such a good job that Kyoshi didn't have to deal with any (+)

• Wasn't able to deal with the rising crime in the Earth Kingdom and in the southern seas (-)

Kyoshi:

• Defeated the daofei (+)

• Caused circumstances that made the Fire Nation able to start it's process to becoming an imperial super power that would take over the entire world (-)

• Defeated Yun (+)

• Created an awful anti-corruption task force, that was corrupt (-)

• Created the Kyoshi warriors to help women (+)

• Killed Chin the Conqueror (+/-)

• Created Kyoshi Island (+)

• Stopped the Ba Sing Se revolt (+)

• Created the Dai Li (-)

• Implemented bureaucratic policies that strengthened the Earth Kingdom (+)

• Created a new constitution (+)

Roku (this might be a little bit retconned soon):

• Displaced people through him causing volcanic activity while trying to get into the Avatar State (-)

• Rebuilt places for the displaced people (+)

• Stopped a war between the water tribe and Earth Kingdom (+)

• Didn't stop dragon hunts (-)

• Didn't stop dark spirits int he north and south (-)

• Didn't stop the cold war between Omashu and Ba Sing Se (-)

• Didn't try to curb xenophobia in the Earth Kingdoma and Fire Nation (-)

• Didn't stop the game of thrones going on in the Fire Nation (-)

• Didn't stop Sozin from waging war on the world (-)

Aang:

• Stopped Zhao’s fleet (+)

• Kicked Long Feng out of office (-)

• Failed at stopping Azula from taking over Ba Sing Se (-)

• Ended the 100 year war (+)

• Solved the Yu Dao crisis (+)

• Tried to destroy the refinery, and thus people's livelihoods (-)

• Killed General Old Iron (-)

• Stopped Azula from kidnapping more children (+)

• Helped with Gilak situation (+)

• Helped put down Yaling (+)

• Created Republic City and the URN (+)

• Took away Yakone's bending (+)

• Let Yakone escape (-)

Korra:

• Stropped Amon from taking over Republic City and equalizing the world (+)

• Gave back all the good benders their bending back after Amon took it away (+)

• Opened and kept the spirit portals open (+)

• Stopped 10,000 years of darkness (+)

• Absorbed both Raava and Vaatu, in theory balancing the Avatar (+/-)

• Helped the Southern Tribe become independent (+)

• Stopped Hundun (+)

• Helped bring back the air nation (+)

• Couldn't stop Zaheer from killing the earth queen (-)

• Defeated Zaheer and the heads of the Red Lotus (+)

• Couldn't stop Kuvira from taking over Zaofu (-)

• Rescued Wu from being kidnapped (+)

• Stopped Kuvira (+)

• Helped destroy down town Republic City (-)

• Created a new spirit portal (+)

• Ended a Turf War (+)

• Stopped Republic City citizens from being poisoned (+)

• Helped turn the Earth Empire into a democracy (+)

• Stopped Guan (+)

• Made LGBTQ rights be better in the Avatarverse due to her publicly known relationship with Asami (+)

The actions don't lie. Korra is not the worst. So then why do people think she is?