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‘ ‘Take it,’ he said, and he thrust the sack into George’s hands. ‘What?’ said Fred, looking flabbergasted. ‘Take it,’ Harry repeated firmly. ‘I don’t want it.’ ‘You’re mental,’ said George, trying to push it back at Harry. ‘No, I’m not,’ said Harry. ‘You take it, and get inventing. It’s for the joke-shop.’ ‘He is mental,’ Fred said, in an almost awed voice. … ‘Harry – thanks,’ George muttered, while Fred nodded fervently at his side’

It’s why they agreed to his request Ron get some new dress robes out of it. They’re clever not just funny ‘they always get really good marks’ but as OP says they’re not as good as Percy, Bill or Charlie. They helped Ron get him out of the Dursleys:

‘But you can’t magic me out either –’ ‘We don’t need to,’ said Ron, jerking his head towards the front seats and grinning. ‘You forget who I’ve got with me.’

They get him into Hogsmede , they (unknowingly) helped the trio break into Umbridge’s ministry office. They liked Harry for himself 

‘This is all your fault,’ George said angrily to Wood. ’“Get the Snitch or die trying” – what a stupid thing to tell him!‘’

And cheered him up when things went wrong, such as Harry being accused of being the Heir of Slytherin

‘They went out of their way to march ahead of Harry down the corridors, shouting, ‘Make way for the heir of Slytherin, seriously evil wizard coming through …’ Percy was deeply disapproving of this behaviour. ‘It is not a laughing matter,’ he said coldly. ‘Oh, get out of the way, Percy,’ said Fred, ‘Harry’s in a hurry.’ ‘Yeah, he’s nipping off to the Chamber of Secrets for a cup of tea with his fanged servant,’ said George, chortling ‘

Or when Ron and Hermione were made Prefects and Harry felt left out:

‘Yeah,’ said Fred slowly. ‘Yeah, you’ve caused too much trouble, mate. Well, at least one of you’s got their priorities right.’ He strode over to Harry and clapped him on the back while giving Ron a scathing look.

They tricked Dudley because they know how crappy Harry’s home is: 

‘We didn’t give it to him because he was a Muggle!’ said Fred indignantly. ‘No, we gave it to him because he’s a great bullying git,’ said George

And of course:

‘Give her hell from us, Peeves.’ And Peeves, who Harry had never seen take an order from a student before, swept his belled hat from his head and sprang to a salute as Fred and George wheeled about to tumultuous applause from the students below and sped out of the open front doors into the glorious sunset. 

Harry frequently heard students saying things like, ‘Honestly, some days I just feel like jumping on my broom and leaving this place,’ or else, ‘One more lesson like that and I might just do a Weasley.’