NiKo is close to S1mple's skill in some areas, but S1mple in addition to having better aim than NiKo with rifles which is NiKo's strongest suit, also has buckets of unprecedented skill in other places where NiKo doesn't even have places. NiKo's aiming STYLE is something different from skill level which NiKo's level is very high, but S1mple is better. S1mple is a high sens player though which is stark contrast to NiKo so his crosshair movement naturally won't LOOK as 'clean' as NiKo's, but that does not mean it isn't as effective. Fact is S1mple has the best aim in Counterstrike as evidenced by having the lowest average time-to-damage in the entire scene. And the crazy thing is this is only one fraction of his skillset. Most top players have one or two things they are very good at, or among the best in the world at. NiKo rifle aim, Devve awp skill, Xyp clutch situations, etc. However with S1mple he has ALL those skills at such a high level that are equal or better than the very elites that specialise in each of them. His movement both Macro and Micro, aim, gun skill with every single weapon, game sense and big picture understanding of a match, and so many other aspects of his skill, are the best in the world, and this is what puts him on a different planet from A NiKo, Dev1ce, Cold, etc. at least in 2018. Even just looking at his stats tells you enough. Leader by a clear margin inn opening kills and kpr while being only one 1vX being Xyp for most clutches.
TL:DR- S1mple's skill as a whole is not only significantly higher than NiKo's, but its also 1000x broader.