The intentional stutter on the g in generation, as well as being present in some other words and verses, is meant as a kind of self - referential mocking of what someone hopped up on amphetamines sounds like.

It would be a fairly obvious and humorous reference to what many concert and festival goers would be used to. Stuttering and slurred speech were/are common effects of significant usage of the kinds of drugs they'd all be familiar with.

https://faroutmagazine.co.uk/roger-daltrey-stutter-the-who-song-my-generation/

The Who’s manager at the time was insistent that Daltrey mimic Hooker’s stammering vocal. “Kit Lambert came up to me and said ‘STUTTER!’,” Daltrey recalled. “I said ‘What?’ He said, ‘Stutter the words.”

When the singer asked for Lambert’s reasoning as to why he should act out his speech condition in a song, Lambert explained, “It makes it sound like you’re pilled”. He was referring to the trend among the mod subculture with which The Who identified in the 1960s to get hyped up on amphetamine pills before a night out. Daltrey joked that he was already on the pills and followed his manager’s orders.

On its face, without stuttering, the lyrics read as a rebellious screed of the optimistic youth against a corrupt and unfulfilling system.

But the intentional stuttering and slurring makes the whole thing into a tongue in cheek, semi self aware critique of the prevalence of massive drug use amongst the people purporting these ideals, making the song to sound more like a mockery of, or perhaps lament of drug addicts with no productive life who are just coming up with an ideological excuse/explanation for their lifestyle.