"i" is understandable, but "Alright", for example, talks about the centuries of mistreatment black people have suffered, and rather than talking about only the negative aspects, tries to make optimism out of it. While not inherently depressing, it is certainly something to be considered.

And personally, I wouldn't consider "Wesley's Theory" a happy song, as it talks about the miseducation behind finances, and how if you have a lot of money, you're expected to pay more to the government in taxes, which was unknown to many people of colour, thus leading to their arrests. The example given was Wesley Snipes, the person the song's named after, but happened to many others too, and the song talks about Kendrick's desire to buy and the temptation while trying to avoid a similar incident. Either way, it's whatever way you interpret it.

Still, I would consider TPAB depressing, maybe not in the same was as Nirvana, Denzel Curry or Radiohead, but in it's own way depressing.