"The refs are never an excuse, but I'll probably be fined for saying this, but it was pretty bad out there," the second-year quarterback told reporters. "Kinda ticks me off."
The Browns were called for nine penalties worth 83 yards in the game, but one call in particular drew their ire when Cleveland's Morgan Burnett was called for a horse-collar tackle during what would be the Seahawks' game-winning drive midway through the fourth quarter.
Mayfield also didn't want to use his "tender" hip as an excuse. The 2018 top overall draft pick briefly went back to the locker room in the third quarter after appearing to hurt his hip on a scramble but returned to the game.
The Oklahoma product finished with 249 yards, one touchdown and three interceptions. He carried the ball four times for 35 yards and his first career rushing touchdown.
His three picks brought his season total to 11, the most by a Browns quarterback through six games since 1984 (h/t ESPN Stats & Info).
It was a frustrating day all around for the Browns. Cleveland jumped out to a 20-6 lead the first quarter but was outscored by Seattle 26-8 from the second quarter on.
"Any loss hurts," Mayfield said. "Losing at home sucks. ... Especially when it was a game we felt like we needed."
Mayfield added: "If we don't hurt ourselves, this team can go where it wants to go. ... If we eliminate that, we can be a great football team."
To Mayfield's point, the Browns entered Week 6 as the second-most penalized team in the league.
Cleveland was looking to rebound from a brutal 31-3 Monday night loss to the San Francisco 49ers. Instead, the Browns lost their second in a row and dropped to 2-4 on the season.
It won't get easier for Cleveland after next week's bye, either, as they travel to face the undefeated New England Patriots in Week 8.