Yes, the Kosovan declaration of independent was just and in accordance with international law, because international law carries a preference for self-determination. International communities believe that each local ethnicity should be able to set laws and governance according to their own customs. In keeping with this, there is nothing wrong with Kosovo declaring independence.
A majority of Kosovars voted for independence, so it is legitimate. But on the flip side, it would be legitimate for the Serb minority to declare independence - or independence and a request for union with Yugoslavia (Serbia). As far as I know, there is no international restriction on independence,of if the UN did botch the Pakistan/India "devolution.".