Maybe it's just not a translation literal enough, even if the meaning is similar. Actually, maybe not even that similar. "This journey WAS a mistake" would be similar because it turned out that it was a mistake (because we failed), but "This journey IS a mistake" sounds like "we WILL regret it because we WILL fail" said at the beginning of the trip by some sceptic. Or maybe I'm just overthinking as usual. Anyway, mistake = błąd, and porażka is a failure or a defeat (as in sports, for example).
Another, more colloquial translation for the original sentence: "Geeee, this journey sucks", in a proper context (especially kids/teenagers complaining). Especially if you add 'jakaś' to the sentence: Ta podróż to jakaś porażka =='
Hi Jellei, "This trip sucks" makes sense and sounds like everyday English. "this trip is a failure" makes sense too, but "this trip is a defeat" is something I don't think would be said in English.