Without taking a position on Catalonia, I think one has to analyze the reasons they want to hold the referendum and decide if you find them compelling. When the USA seceded from the UK, the Declaration of Independence provided a reasonable list of reasons. By contrast, the main reason the CSA seceded from the USA - slavery - prevented their cause from being legitimate.
Independentism is always a thorny subject, because most justification ends up being defined ex post. It’s like “resistance” movements against lawful but tyrannical governments: if they succeed they will be freedom fighters, if they fail they’ll be brigands and terrorists. Usually the threshold for legitimacy is overwhelming support from the population, something that happens sporadically (it happened in Slovakia, Slovenia, Croatia, Kossovo, almost in Scotland, and of course in post-soviet republics all over the old federation). But once popular support is somehow made manifest, then the Wilson doctrine must be allowed to set the agenda.
Invading a country to abolish slavery is morally justifiable, invading a country because you want to tax them isn't.