National liberation is not the same thing as separatism. Sometimes separatism and national liberation are at odds with each other. The separation of the USSR's national republics from the union was not liberation, it was the opposite. Same for the Yugoslav republics. It exposed those republics to imperialist subjugation and neo-colonial plundering. On the other hand, the success of the Scottish national separatist movement in the UK for instance would severely weaken one of the main imperialist powers and would open up possibilities for more a more progressive path for a Scotland free of the British monarchy.
There is no one size fits all answer. Whether or not you should support this or that separatist movement depends entirely on the circumstances and the context. Would the success of said separatist movement represent a real liberation or merely a balkanization in the interests of greater imperialist powers? In my opinion this matters more than whether the separatists themselves are right wing or left wing.
Think of it like the difference between intentions and actual consequences. It doesn't matter that your intentions are good if your actions have objectively bad consequences, and conversely, people with bad intentions can end up doing things which have good consequences in the broader context, even if they did those things for the wrong reasons. We always say Marxism is not about moral judgements, it's about looking at the world with a sober materialist analysis. You should not look for an intrinsic moral value to separatist movements but instead consider their real material effects on the world. Who benefits?