Basically, Sassuolo is a team that quite often produces unexpected results when playing against top Serie A teams. But unfortunately, this season Sassuolo failed to do this so Sassuolo is also in the relegation zone this season. But indeed, Sassuolo is a mediocre team that managed to beat Inter this season even though it is true, in reality in that match Inter also had high dominance but Sassuolo still managed to get three points. It is also possible that Sassuolo is really still making big efforts at this moment to get out of the relegation zone. Because at least, if Sassuolo manages to be consistent with three points for the remainder of this season, then maybe Sassuolo will manage to get out of the relegation zone, who knows.
To be fair, even though nobody really expected this result, I did expected an "unexpected result" to happen with Inter eventually. Because they already won, so there isn't really anything left to fight for, and they could just end up with not caring about it at all.
I understand that it is not going to be all that simple, we should consider this fact as something that wouldn't be a big deal. Just accept that Inter doesn't care about it anymore and that's why we got this result. It wasn't some great deal from the opponent, if Inter cared enough about to win this game, they would still win it, they are capable of winning it, they just didn't care. More like this could potentially maybe happen, because the league is over and they just do not care at all.
For the second time this season, Sassuolo managed to bring Inter to their knees before them, a result we never imagined would happen. We know that Inter basically doesn't care about the results anymore, because they have confirmed themselves as Serie A champions this season. I think many bettors experienced losses in that match, because they were so confident that Inter could secure full points, maybe in the next 3 remaining matches betting on Inter winning requires a lot of consideration. Even though we know that Inter are the favorites in every match, in reality they are treating the remaining competition in a relaxed manner and are no longer forcing the best results.
Meanwhile, for a mediocre club like Sassuolo, an additional 3 points means a lot to survive in Serie A next season. Sassuolo still has a chance to save themselves from the relegation zone, but that depends on the results they can achieve when facing Genoa, Cagliari and Lazio. If Sassuolo succeeds in sweeping all the remaining matches and securing full points, then they have the chance to survive in the Serie A, because the three clubs above them have difficult matches ahead.