Kovalchuk tied his career-high in goals with 52, matching his total for the 2005-06 season. His performance has been one of the few bright spots for the Thrashers, who are in 14th place in the Eastern Conference.
"I just want to forget [the season] like a bad dream," he said.
Kovalchuk has been expecially good lately. He has 16 points (11 goals, five assists) in 15 games during March. He also has eight goals in the past seven games against Tampa Bay.
"He's had a great season," Thrashers coach Don Waddell said. "He carried this hockey club for a couple of months early in the season. He's a goal scorer. If he gets opportunities, he's going to bury them."
Kovalchuk scored into an empty net with 58 seconds remaining and Lehtonen picked up his fourth shutout of the season.
Smith, who made his first start in three games due to a sprained left knee, had 23 saves on 24 shots.
The Thrashers stayed out of last place, which is held by the Lightning with 71 points. The two teams were scoreless through 55 minutes, despite Tampa Bay having five power-play opportunities in the first two periods.
"It's embarrassing to me, for both teams," coach John Tortorella said. "Don't talk to me about effort."
Lehtonen found little comfort in staying out of last place.
"I haven't looked too much at the standings in the last couple of weeks," Lehtonen said. "It really doesn't matter if you're in last or second to last."
"Both teams were playing for pride," Waddell said.
Lightning defenseman Dan Boyle said Monday summed up an awful season for Tampa Bay as well.
"[The season] has got to come to an end at a certain point," Boyle said. "I think tonight symbolizes, to me, a lot of what's happened this year where we played hard for most of the game and we don't capitalize. [Then] the other teams score late in the third and they win the game.
"Tonight was a pretty good illustration of what's happened this year."
Thrashers LW Jordan LaVallee made his NHL-debut Monday, after being recalled from AHL Chicago Wolves on Sunday. ... Atlanta defenseman Ken Klee missed his sixth game with a groin injury. ... The Lightning are 0-6-1 in their last seven home season finales.