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Slytherin ideals aren't about being mean, it's about being cunning and sly, which Slughorn was to a T with his slug club shenanigans.
All of these are bad examples, or rather counterexamples.
Hufflepuff just takes "the rest." The misfits and outliers, the mediocre and the moderate. These can be great wizards and witches, or they can be utter shit. That's Hufflepuff.
Lockhart was exceptionally smart and talented, but he was a spineless weasel and gloryhound. He used his skills as at obliviation to alter memories and make people love him. He was a leech and a criminal, and inept at other magical skills, but he made himself rich, famous, and universally beloved while avoiding any actual practice or hard work. He was probably a very gifted student who figured out how to get other people to do his work and make him look good. That's why he was ultimately inept.
Pettigrew was a follower. No doubt he did not fit the mould of the typical Gryffindor in that he fell to corruption and betrayal, and managed to hide as a rat for decades. But remember, the sorting hat takes your choice into consideration. He likely wanted to be a strong and brave leader. He wanted to be cool like his friends, and he was not. He was small and scared, and always reminded of it. After school, his friends weren't there to protect him and make him feel special. He wasn't one of the "in" crowd anymore. He was a ripe target for a cult that brought him in and made him feel powerful again.
I'm like 99.9% sure that's not the guiding principles of Hufflepuff.
Yes that's a song he sings because it rhymes. At other times the actual criteria are listed and it's not just the leftovers.