Is there any evidence in the Bible to say that the rest of Seth's ancestors were righteous rather than just being sinful like everyone else?
Potential Answer: Embedded at the end of both the genealogies are poetic references to what appear to be a pre-Noahetic bronze age. One ancestor Lamech from the Cain side brags to his wives that he can use this new found technology that his son has developed to fashion weapons and kill young men; the other Lamech on the Seth side looks as though he is thankful that this new technology can be used to fashion tools to farm with:
Gen 4: 22 Zillah... also gave birth to Tubal-cain, the forger of all implements of bronze and iron... Lamech said to his wives, “Adah and Zillah, Listen to my voice, You wives of Lamech,Give heed to my speech, For I have killed a man for wounding me; And a boy for striking me; 24 If Cain is avenged sevenfold, Then Lamech seventy-sevenfold.”
Gen 5:28 Lamech lived one hundred and eighty-two years, and became the father of a son. 29 Now he called his name Noah, saying, “This one will give us rest from our work and from the toil of our hands arising from the ground which the Lord has cursed.”
The embedded poem and phrase in the genealogies underscores the differences between them, one side using this power for food, the other to kill. One remains somewhat God centric in thought abet negatively the other indicative of self centric hard hearted boasting over death.
What could have triggered the flood was the intermarrying between these two morally divergent genealogical lines.
Son's of God taking the daughter's of men
There is a defense for the "sons of God" being Cain side human rulers of the time, they intermarried with the Seth side and so destroyed that side's morality, plunging all humanity into a degraded state that precipitated the great flood. See Bruce Waltke's Commentary on Genesis for more information on this.
As far as Noah's ancestor is concerned, maybe not affected by the intermarriage morally. His death possibly prophetically releases the flood. (going from memory) I think inbedded in his name is the idea that judgement will not come as long as he was living; he lived longer than any other pre -flood individual, suggesting a similar mercy at work today i.e. patience of God at work postponing a second judgement.
shorthand answer: The moral diversity of the lines of Cain and Seth seems present imbedded within the genealogies themselves, the extinguishing of righteousness is a result of later intermarriage culminating in judgement right after Methuselah dies, thus keeping congruent the fraze "Noah the only righteous man."