Hot lunch is a pun, a play on words.
All the OP needs to know is that the protagonist of the clip is Prince Gristle. What is a gristle? It's a stringy piece of meat elastin that is particularly unpleasant to look at, and more significantly, tough to chew. If you find gristle in your meat, you'll probably feel mild disgust and leave it on your plate.
From the following Wikipedia synopsis, we discover that Prince Gristle is a Bergen, the term sounds very similar to burger, and sub-par burgers will contain bits of gristle. Bridget says that Prince Gristle looks fat, which he is, but immediately corrects herself by adding P-H, Phat. Phat could be an acronym that stands for Pretty Hot and Tasty, but Merriam-Webster defines it as very attractive or appealing, and Oxford Dictionaries says it's an informal term for excellent.
Thus we have names and expressions that evoke the theme of food: Gristle, Bergen, and phat. When the Prince exclaims "hot lunch" the theme of food has been laid out on the table. Attractive and sexy "babes" (and "hunks") in the US are often described as being hot,
Merriam-Webster defines it as sexy in its second definition
2. b (1) : sexually excited or receptive It's obvious he's hot for her.
(2) : sexy That guy she's dating is really hot.
While Oxford Dictionaries describes hot as someone who is sexually attractive and offers this example, ‘a hot chick’.
So in the movie, hot lunch is just a punchline, a gag, and, as @1006a noted, a phrase that the movie's target audience—school kids—are largely familiar with. English speakers do not normally call anyone sexually attractive a hot lunch, but they might say someone is tasty looking and the phrase "I could eat you up" is said teasingly to someone who looks very cute.
The plot synopsis courtesy of Wikipedia
The Trolls are small creatures who live in an almost perpetual state of happiness, singing, dancing and hugging all day long. However, they are discovered by the Bergens, large creatures who never feel happy, but discover that they can feel happy for a moment, if they devour a Troll. The Bergens put the Trolls and their tree in a cage, and hold an annual festival, called Trollstice, in which each Bergen gets its feeling of happiness, by eating a Troll. However, the Trolls, led by their King Peppy, escape through underground tunnels on the day that Prince Gristle Jr. would eat his first Troll.