Whether or not it is the correct legal decision depends on whether you think judges should have personal input into whether election law should make sense or not. The three liberals are clearly concerned about the possibility of GOP states ruling that all Dems are insurrectionists based on nothing and keeping them off the ballot (though one would think that SCOTUS could check that by determining whether or not someone actually committed insurrection) and see a real judicially cognizable interest in supporting some degree of uniformity in elections for federal office, but this decision isn't remotely based in the text or history of the 14th Amendment nor in structural Constitutional concerns (typically the Court looks at federalism, especially in elections, as good, not a horrible thing to be avoided at all costs even to the extent someone who committed treason should stay on the ballot).
If you're a legal realist that thinks the courts have a role to play in filling in the blanks when the text of the law isn't entirely clear, I guess it's not insane for you to think that this is the correct legal decision (though I think the text of the law is fairly clear here). But if you've ever spent even a single fucking second touting anything any conservative legal commentators have said about what law should be and you want to pretend that this aligns with what you've previously touted, you should ___________________________________________________ [content that would get me suspended and/or might encourage someone to actually commit self harm has been censored by me].
A SCOTUS ruling that Colorado's electors can't be counted. Based on what? Who fucking knows, but that's what would happen.