Brett Ritchie also scored for the Stars, who won for just the fourth time in their last 38 trips to Philadelphia.
Luke Schenn scored for the free-falling Flyers, who lost their third straight to tumble further out of playoff contention. The loss snapped a six-game home point streak for Philadelphia, which lost at home in regulation for just the second time in 13 games.
Fiddler scored the game-winner with about 6 minutes left in the third period. Colton Sceviour centered a pass from the right boards and Fiddler redirected it past Steve Mason from the goalie's doorstep to give Dallas a 2-1 lead.
Kari Lehtonen made 24 saves to earn his first career victory in 14 games against the Flyers, improving to 1-10-3. Lehtonen's best save may have come in the second period when he denied Jakub Voracek from point-blank range.
It was another blow to the Flyers, whose slim playoff chances took a major hit over the weekend. It began with Saturday's overtime loss to Boston when Philadelphia gave up the tying goal with 14.1 seconds left in regulation. That was followed by Sunday's 5-2 defeat at New Jersey.
The Flyers began Tuesday in 11th place, seven points behind the Bruins for the Eastern Conference's final playoff spot.
Dallas was farther back in the Western Conference, trailing Winnipeg by 10 points for the last spot.
Schenn opened the scoring with a slap shot from the point that went over Lehtonen's glove to put Philadelphia in front 1-0 just 1:13 into the contest.
Dallas evened it at 1 with 8:37 remaining in the period when Ritchie fired a wrist shot between Mason's legs from the slot.
Tyler Seguin, among the league leaders with 31 goals, was playing in his second game after missing 10 due to a sprained knee. He scored two in Saturday's 5-4 loss to Tampa Bay but didn't have a point against the Flyers.
Philadelphia's recent struggles on the power play continued, as the Flyers went 0-for-3 on the man-advantage. The Flyers, who entered third in the NHL in power-play scoring (23.1 percent), have scored just once on their last 15 power-play opportunities.
Notes: Flyers C Vincent Lecavalier, a healthy scratch for seven of the last nine games, returned to the lineup while LW R.J. Umberger was a healthy scratch. Umberger has gone 18 games without a goal. ... Stars D Jason Demers (sick) wasn't in the lineup. ... Flyers D Nicklas Grossmann (upper body) returned after missing four games. Grossmann took a puck to the face on a second-period shot by Antoine Roussel but remained in the game.