Dealing with depressed people is a burden. Trying to lessen the seriousness of it is escapism- and at that point you should probably write about something else, no?
What you can do, is make something else about the story cute or not so serious. A good example might be Ancient Magus Bride... Maybe.
I don't really get your point. Trying to make depression cute or funny is counterproductive to your stated goal. If you genuinely want to help, then don't belittle the situation.
If your story is a cute romp through the pixie filled flower fields and unicorn pastures, then let depression be the dragon. The one thing that's deadly serious in the entire story.
I guess I don't count as normal because I minored in psychology. All the same, depression is sometimes caused by circumstances but often a simple imbalance of neurotransmitters. If you know the circumstances, it's easier to avoid talking about those things.
If you're not close to the person, they often won't even tell you about their depression in the first place. If they tell you, you can ask them if there is anything that would make interacting with you more enjoyable, any triggers to avoid. You can also apologize for not knowing much about depression and invite them to stop you from talking if you get into sensitive territory.