Where Tiefling "often" disagree with the Evil alignment of their plane, implies that many other Tiefling are fully Evil like their plane, and remain there, perhaps even more often.
Meanwhile, those Tiefling who grow up elsewhere are "any" alignment. Some of them will be Evil, and "often" members of an Evil faction corresponding to the individual Fiend that originated their lineage.
Again.
No.
What does the text ACTUALLY say? What is the tone Wizards ACTUALLY conveys? Its not even difficult, you just continue to interject your own head canon or definitions or intent, where none exists. So, here we go.
Tieflings born in the Lower Planes often migrate to other planes of existence and never look back.
There is no interpretation needed here. Those born in the Lower Planes often migrate out, and never look back. There is no value judgement. There is no explanation, there is no nuance. There is no qualitative here beyond 'often'. Wizards is saying 1 thing, and 1 thing only.
Of those Tieflings born in the worst places in the multiverse, they OFTEN leave. End.
Their descendants are scattered across many worlds and are especially prevalent in Sigil, the torus-shaped city at the center of the multiverse.
Again, this is not open to interpretation. Tiefling descendants are on many worlds, are are especially found on Sigil, which is the center of the multiverse.
Thats it. No judgment. No moral question. No Alignment. Nothing but that. End.
The earliest tieflings joined ranks with non-tieflings to repel fiendish incursions on many worlds, earning the trust of those who might otherwise have mistaken them for Fiends.
Finally, something!
They joined ranks with other species to repel fiendish incursions on many worlds.
They earned the trust of those who might otherwise have mistaken them for Fiends.
These are 2 direct, positive statements. "Joined Ranks" and "earned the trust", are both contributing to the POSITIVE TONE. End.
Thanks to the victories and sacrifices of these legends, tieflings throughout the multiverse enjoy widespread acceptance.
And the peak of nuance (aka subtlety) here.
Thanks to the VICTORY and SACRIFICE of these LEGENDS, tieflings THROUGHOUT the multiverse ENJOY WIDESPREAD ACCEPTANCE. END
This cannot even be possibly mistaken for anything but a statement of positive glory for the entire species. At all. If I were to read this, I would assume they are one of the most noble species in the setting!
This is about as nuanced as a hammer to the side of the head. If you gave this to anyone, told them what the "Lower Planes" are, told them Tielfings are born with a touch of this "Lower Planes" in their very essence, and then asked them if Tieflings are good or bad based on the actual Wizards text?
"They are good people who fought against evil and enjoy widespread acceptance."
That is it, that is all. That is what Wizard has SAID. Anything else is your head canon as such as it refers to this Origin UA description.