The question has been asked about Jesus (and others, a few times) in the pages of the Bible saying something like "There is no one good but God..." as Jesus does in Luke 18 and other places where Jesus is speaking to the "rich young ruler" (who approached Jesus, saying, "Good teacher, why...")
First of all, we have to note at least two things:
1. Jesus (and other biblical authors/speakers) refer to good people. "The sun shines on the evil and the good," for instance. But many other instances, as well. Jesus clearly thought there were good people.
2. The reality that, in spite of what the Bible says, of course, there are good people as good is typically defined/understood. There ARE good people definitionally, in the real world.
Now, dealing with the passage in question. In Luke 18, Jesus is in the context (here and throughout his ministry) of dealing with the legalism and gracelessness of the Pharisees. Jesus even offers a parable of the Pharisee and the Tax Collector:
To some who were confident of their own righteousness and looked down on everyone else,
Jesus told this parable:
“Two men went up to the temple to pray, one a Pharisee and the other a tax collector. The Pharisee stood by himself and prayed: ‘God, I thank you that I am not like other people—robbers, evildoers, adulterers—or even like this tax collector. I fast twice a week and give a tenth of all I get.’
“But the tax collector stood at a distance. He would not even look up to heaven, but beat his breast and said, ‘God, have mercy on me, a sinner.’
“I tell you that this man, rather than the other, went home justified before God.
For all those who exalt themselves will be humbled,
and those who humble themselves will be exalted.”
Following that parable, Jesus reminds everyone (with an eye to the Pharisees who he JUST addressed):
Truly I tell you, anyone who will not receive the kingdom of God like a little child will never enter it.”
He said that in dealing with the arrogance and gracelessness and self-perceived "righteousness" of the Pharisees.
In THAT literal, specific context of the proud, arrogant, graceless, legalistic Pharisees, the story moves to a rich ruler approaching Jesus - another of the typical antagonists in Jesus' life and message... the OPPOSITE of the humble poor and marginalized that Jesus told us he'd come to preach good news to (and in contrast to the wealthy, powerful and arrogant, who would be "brought down" in Mary's Magnificat and other places). The rich ruler talks and Jesus answers:
A certain ruler asked him, “Good teacher, what must I do to inherit eternal life?”
“Why do you call me good?” Jesus answered.
“No one is good - except God alone.
Then, when the rich ruler tells Jesus (rather arrogantly!) that he's kept all the rules since he was a boy (!!) to THAT man (and by extension, the other arrogant legalists like the Pharisees) and THAT man specifically, Jesus says:
“You still lack one thing.
Sell everything you have and give to the poor, and you will have treasure in heaven.
Then come, follow me.”
Jesus using the hyperbolic saying "there is no one good but God" is addressing the arrogant rich and powerful ruler, along with the Pharisees in the crowd and for the benefit of the poor and marginalized people that Jesus came to preach good news to. Indeed, for those humble poor and marginalized folks, THIS response of Jesus WAS good news. "He's taking on the man! He's bringing down the arrogant and powerful rich ones!"
The marginalized poor folks were well aware that they weren't perfect... that they fail to hit the mark. They understood/understand imperfection and the flaws of humanity. They know that they are not perfectly good like God. But the rich and powerful, the arrogant and legalistic like the Pharisees, THEY often didn't know this, blinded as they were by their wealth and privilege.
In the text and context of this story, Jesus makes clear who he is addressing with his "No one is good" hyperbole...
To some who were confident of their own righteousness and looked down on everyone else...