So for the technical variants it would mostly be a question mof fidelity. For the style variants, you could have a bunch of different judging criteria, but in

So for the technical variants it would mostly be a question mof fidelity. For the style variants, you could have a bunch of different judging criteria, but in the end, yes, very subjective, and the panel of judges would give ratings, drop the two highest and the two lowest, average the rest. You could even constrain this to matching the genre/style of the original, or specifying a genre/style for the performance independent of that of the original. Or could just go head-to-head and let the judges decide rather than trying to give an objective ranking. If a host country chose "Mary Had a Little Lamb" as their "song by a living artist" then this competition is taking place in the 1830s and we'd probably need to address that first.