The only reason Ash didn't sweep Nando is because he used wasn't using more powerful Pokemon, which he could have easily done. Ash would have swept Nando 3-0 singlehandedly using Infernape, let alone Sceptile who was even stronger. As for Macy, he would have swept her if he actually used the stronger Pokemon that he owned at the time (i.e. Charizard, Snorlax, etc).
One thing people forget is that if Ash switched out Gliscor for Infernape after beating Drapion, then Ash would have actually beat Paul of all trainers 6-3 without needing Infernape's Blaze at all. The Ash vs Paul battle was only 6-5 because Ash kept out Gliscor, then sent out Pikachu, and then deliberately triggered Motor Drive because he tried to play 4D Chess using Pikachu and it backfired spectacularly. The writers love having the winner of a battle being pushed to their last Pokemon, even when said trainer would win 6-4 or 6-3 (or 3-1 or 3-0 in a 3v3 battle) by conserving their remaining Pokemon.
As for Guzma, if Torracat is capable of pushing Golisopod into using Emergency Exit then beats Scizor right afterwards then no, Guzma isn't stronger than Nando. Remember that Nando had to battle three of Ash's Pokemon: Quilava is stronger than Torracat, Heracross is much stonger than Torracat, and the context of Staraptor's win over Paul's Weavile also puts it above Torracat. This is because Staraptor basically tanked a Stone Edge from Paul's Torterra (stronger than any Pokemon belonging to anyone in the Alola League), beat Weavile at a type disadvantage despite taking massive damage from Torterra, and was still able hit Ursaring once before finally going down.
Despite being part Water-type, Golisopod is going to lose at least half its health, then Scizor will come out and get beat by Torracat who already had to battle Guzma's strongest Pokemon. The fact that Golisopod is struggling against a Pokemon it has a big advantage over suggests that Guzma, while stronger than everyone except the semi-finalists, isn't actually top tier. It makes me believe Golisopod's fight against SM Pikachu was actually just a massive outlier whereby Pikachu was the one nerfed for that battle.
TL;DR: Ash is very much capable of sweeping trainers much stronger than Guzma, and doesn't sweep often because he choses to use weaker Pokemon. Nando is actually a better trainer than Guzma overall, and all three of the Pokemon Ash used against Nando were stronger than Torracat.