Apart from consistency with other 'cum in X' tags, I think it's described as being 'in' the nose because it would've entered the nasal cavity in the first place in order to be visible leaking out. But it might not always leak out; it's possible, for example in a cross-sectional 'X-ray' view, that cum would be depicted having entered the nose but not be forceful enough to push all the way through and out. Cum flowing out from the nose is merely a symptom of it being in the nose, if that makes sense.
In any case if the tag were renamed or aliased I'd suggest calling it cum FROM nose. Cum out nose doesn't parse well, like it should be 'out of' or 'out from'. Using 'out' without qualification suggests more of a past-tense setting of something having been extracted fully and deliberately from the nose, more comparable to the presence of a dirty handkerchief in the absense of ongoing sneezing or blowing.