The midfielder was dismissed eight minutes from the end as the Canaries drew 0-0 with Newcastle, with Loic Remy also sent off for his part in the altercation
Johnson was sent off eight minutes from time following an altercation with Loic Remy, in which the Norwich midfielder shoved the Frenchman in the chest before Remy reacted by moving his head towards the 26-year-old.
QPR loanee Remy was also shown a red card for his part in the incident, but Hughton did not believe that Johnson's dismissal was justified.
"I had a very good view of the incident where Remy leant his head into Bradley Johnson," he said. "But probably more for me to speak about is the Bradley Johnson sending off, which I thought was incredibly soft.
"I presumed when I saw he'd been sent off that something else had happened, apart from what I did see, which was him push Loic Remy in the chest. But no, apparently he was sent off for that incident.
"I don't think he would have been sent off if Loic Remy hadn't have been sent off. So I thought that was incredibly soft."
Norwich found themselves on the defensive for much of the game as Newcastle dominated, with the visitors hitting the woodwork on three occasions.
The hosts rallied towards the end, and would have taken all three points had it not been for a fine save from Tim Krul to deny Robert Snodgrass late on.
"We have to be honest enough to say we didn't play particularly well [in the] first half. I thought Newcastle were very good," Hughton added.
"I thought they were sharper, better on the ball than us and more aggressive than us in that first half period and [we were] probably delighted to come in at 0-0.
"I thought we got to grips with the game [in the] second half. I thought we were better, [and had] arguably the best chance of the game with Robert Snodgrass at the end. Overall it was a fair result."