The natural, instant conclusion is that the defense sucked late in games and should have done better. There's some truth to that, especially earlier in the season when the defense was genuinely bad. But over the second half of the season the problem's mostly been that the offense simply couldn't score. Since week 8, the Bucs' offense has scored just 34 points in the fourth quarter. The defense gave up 52 points over the same period, but 24 of those came in the last two games.
Which is to say that both sides share some of the blame, but I'd certainly be inclined to blame the offense a lot more than the defense, which often forced multiple punts late in games to give the Bucs' anemic offense another chance.
The Bucs were also one of the worst teams in the league in close games, playing 11 games where either they or their opponent had an opportunity for a comeback or game-winning drive -- that trailed only the Arizona Cardinals, who had 13 such games. The Bucs lost all but one of them -- and the one they won came against the Pittsburgh Steelers with less than a minute left, after they'd squandered comeback opportunities.
The Bucs were terrible late in games, and in close games this year. That's the bad news. The good news is that that kind of performance doesn't stay consistent from year to year. Doing poorly in close games is to a large extent just being unlucky. And luck doesn't hold up across multiple seasons.
So there's your reason to be optimistic for next season. Not only have we seen defensive improvement throughout the season. Not only should we expect the offense to get better with a different quarterback and some new players along the offensive line, as well as an actual offensive coordinator. We should expect the team to be better simply because no one can be that bad in close situations consistently.