One common theme is hope. The characters from Undertale often try to bring hope to the depressed reader. For example, Papyrus with his positive attitude might inspire the reader to see the good in things. Another theme is acceptance. The Undertale world is known for accepting all kinds of characters, and in these fanfics, the reader may find acceptance they lack in their real life. It could be that the monsters in the Underground don't judge the reader for their depression.
Isolation and breaking out of it is another theme. The depressed reader may initially feel isolated, much like some of the characters in Undertale. But through their interactions with the game's characters, they start to break free from that isolation. For instance, Undyne's bold and determined personality could encourage the reader to step out of their shell and engage more with the world around them.
One good place to start looking for such fanfics is Archive of Our Own (AO3). There are many talented writers who contribute to this genre. You can search for the specific 'undertale x depressed reader oneshots' tag on the site, and it will bring up a variety of works. Some might focus on how the characters in Undertale interact with a depressed reader - for example, Sans might use his laid - back nature to try and cheer up the reader, or Frisk could offer their unwavering kindness.
Hope is also a very common theme. Despite the deep - seated depression of the reader, there is always a glimmer of hope that is presented in these fanfictions. Whether it's through the simple acts of kindness from a monster like Sans or the grander idea of finding a way out of the Underground with a new - found sense of purpose, hope is what keeps the reader (and the story) going. It gives the message that even in the darkest of times, there is always something to look forward to.
One common theme is redemption. The reader often tries to redeem Sans from his depressive state, giving him a new lease on life. For example, in many fics, the reader's kindness and understanding slowly pull Sans out of his darkness.
In a lot of 'depressed reader' fanfiction, there's the theme of internal conflict. The reader is constantly at war with their own negative thoughts and emotions. There's also often a theme of hidden pain. The reader might seem fine on the outside but is crumbling on the inside. And relationships play a big role too. It could be about the reader learning to trust and open up in a romantic or platonic relationship.
Protection is a common theme. The undertale characters often feel the need to protect the child reader from the various dangers in the underground world.
Romance is also a prevalent theme. The relationship between the reader and Muffet can be a slow - burn romance where they start from a place of suspicion or curiosity and gradually develop feelings for each other. There can also be themes of friendship, where the reader and Muffet form a bond that is not necessarily romantic but still very strong and meaningful.
One common theme is redemption. Toriel, having some regrets in her past, can find a form of redemption through her relationship with the reader. Another theme is discovery. The reader discovers the world of Undertale, and Toriel discovers new aspects of herself.
A common theme is hope. Sans often represents a glimmer of hope for the reader. He shows that there are people (or in this case, a skeleton) who care and can help them through tough times.
One common theme is hope. The Hetalia characters often bring hope to the depressed reader. They show that there are people (or in this case, characters) who care and want to help.
The exploration of morality is a theme that can be found. In Undertale, morality is a huge part of the gameplay with choices between mercy and killing. In BATIM, there are also moral dilemmas in a sense, like whether to trust certain characters or not. Fanfics might combine these moral aspects, having characters from both face new moral choices in the crossover world. Also, the theme of discovery is there, as characters discover the new rules and beings of the other's universe.