The Arsenal forward was left out of Fabio Capello's 23-man party on Tuesday, with Shaun Wright-Phillips and Aaron Lennon picked ahead of him as England's flying wingers.
Walcott was a surprise inclusion in the 2006 World Cup squad having not played in a single Premier League match and looked a certainty for inclusion after firing a hat-trick in a 4-1 win over Croatia in September 2008.
But injuries and inconsistent form have cost him, and Mullery told Sky Sports News: "I didn't think (Capello) would pick Theo Walcott because he doesn't seem to have improved over the last year or so.
"It doesn't surprise me, though it might surprise a lot of people because they will say he has got great pace."
Sir Geoff Hurst, whose hat-trick helped England win the 1966 World Cup, said on the subject of Walcott: "I've seen him play for Arsenal on the box in the latter part of the season, he's had injury problems and I don't think he's replicated his England form towards the back end of the year."
Darren Bent was another man to be omitted as Capello plumped for Wayne Rooney, Peter Crouch, Emile Heskey and Jermain Defoe as his four strikers, and Mullery felt Bent missed out because Heskey's attributes make him better suited to international football.
He added: "It's no disrespect to Darren Bent but it's very rare for him to take up balls being played into his feet and then spinning off defenders.
"You have got to be able to get control of the ball, hold the ball up, and Emile Heskey does that very well. Even though he doesn't score as many goals as Darren Bent he can keep hold of the ball."
Former England midfielder John Barnes felt the inclusion of Bent would have made England top-heavy in the striking department.
"I think we get caught up in how many strikers we want to take, but there is no other country in the world who will take more than four strikers," Barnes said.
"Look at Spain - they pick players for a nice blend and a nice balance to create a certain method of playing, with two or three strikers that will put the ball in the back of the net.
"We could take seven strikers, but if we are not going to play in a way to create chances for them, it doesn't matter how many strikers you have. So I wouldn't necessarily have taken five strikers."
Along with Walcott and Bent, Manchester City winger Adam Johnson, Tottenham centre-back Michael Dawson, Everton full-back Leighton Baines and central midfielders Tom Huddlestone of Tottenham and Scott Parker of West Ham were the other men cut from the 30-man provisional squad.
Hurst felt Johnson would have been worthy of a spot on the place to South Africa after some impressive displays for Manchester City.
"I think there's a place in the squad for Adam Johnson, who had a late importance coming in from Middlesbrough to Manchester City in the latter part for the season," he said.
"I think if he had been involved for Man City throughout the season he'd have been in the squad. He does give us a bit more variation. I think his final ball is better than the existing wide players we have in the squad and I think he probably deserved a place as a latecomer."
Barnes backed the decision to include Joe Cole as he feels England will be best served down the left-hand side through the combination of Cole going forward and his team-mate and namesake Ashley Cole at full-back.
"Joe has re-emerged now, showing his form and quality and I think he would help Ashley Cole," said Barnes, winner of 79 international caps.
"Ashley Cole likes to attack and I think when he has a winger outside him he doesn't get forward as well as when he plays with a left-sided midfield player who comes infield a lot.
"If Joe Cole was to play out there, that would help a lot because it would allow Ashley Cole to be like a winger, which is what we want."
Tottenham manager Harry Redknapp was amazed to learn of Walcott's omission, having expected Spurs winger Aaron Lennon to be competing for selection in the starting XI with the Arsenal player.
"I thought Aaron was a certainty, I thought Theo was a certainty," Redknapp told BBC Radio Five Live.
"I thought both of them would go. I'm surprised Theo's not going.
"He's had a bit of an indifferent season with injuries but he's a good player."
Redknapp backed Huddlestone and Dawson to push for a place in future tournament squads, and never had any doubt Crouch and Defoe would be selected.
However, he thought Bent could have squeezed Heskey out.
"I really felt it was a toss-up between Darren Bent - who's had a great season at Sunderland, been in good form - and Emile Heskey, who's not a good season in fairness, hardly played this season," Redknapp said. "He [Heskey] has played well for England in the past and I think the manager likes it when he plays with Rooney.
"He must have [gone on the partnership] in this situation because otherwise you'd pick Darren Bent. He's had an outstanding season for Sunderland. He's unlucky not to go."