After hitting the winning runs in the first Test, new boy Bethell plundered 96 in the second innings of the second match.
The 21-year-old, who came into the team after Jordan Cox suffered a thumb injury, fell just short of what would have been his first professional century in what is only his 22nd first-class appearance.
And with the decision to place Bethell at three having come under scrutiny before the series started, Stokes feels the youngster has swiftly set about showing just what an exceptional talent he is.
"I'm pretty sure there were a lot of doubters about the role we brought him into for that first Test match," Stokes told BBC Sport.
"You've got a young lad with so much potential and so much talent, why not let him go out there and expose himself to Test cricket at its toughest?
"I was devastated for him to not get that three figures. I said to him: 'It's only four runs, isn't it?'
"His response was: 'Yeah, but it would have been flair if I smacked that through the covers to bring it up.'
"I think he's proved a lot and proved why we rate him so highly."
Stokes does not feel Bethell's performances have provided him with a headache when it comes to selection, though.
"No, I don't see it like that," he said.
"If it hadn't gone well for him in those first two Test matches, that wouldn't have changed anyone's mind in our dressing room of him as a player."
Joe Root plundered his sixth Test century of 2024 in England's second innings, matching the joint-highest total of tons in a calendar year by an England player.
Stokes then took 3-5 with the ball to skittle through New Zealand's tail, after Tom Blundell had offered some resistance for the hosts on Sunday with a brilliant 115.
"In this Test in particular, on day one we were 43 for 4 to be here on the winning side on day three is quite phenomenal," said Stokes.
"We got bowled out in fifty overs (54.5) but we had 270 runs on the board which was a good score on that day one-wicket, and then we obviously had time left with the ball to be able to make some inroads.
"To score 270 and take five wickets at the end of day one really set this game up.
"We back ourselves to be able to go out there and change games and we feel that there needs to be a bit of impetus put into the game, particularly with the bat.
"As a bowler you want to be able to plonk it on a length and plonk it onto the top of off stump over and over again. But sometimes when a batter is brave enough to take you on and knock you off your length, it's very hard to commit to still doing that."
New Zealand captain Tom Latham, meanwhile, credited England's dominant display, and handed particular praise to Harry Brook, who scored 123 in the first innings and 55 in the second.
"It's the pressure they are able to put you under, especially with the bat," said Latham.
"I think on day one it was offering enough for our bowlers and the way Harry played in that innings was outstanding.
"Credit where it's due. The way he played puts you under pressure from ball one. He played some incredible shots that you don't necessarily see from other teams around the world."