Leonardo diCaprio looks less depressing because he obviously prefers all these serious/tragic roles and his climate activism anyway. This guy feels like one of the few who genuinely managed to go from pretty face to serious actor, which hardly anyone else managed to do. I think he still slays very young prime females and the media is regularly attacking him because he still didn't "man up and marry" but instead continues to fuck 20-something models:
For a pretty boy, he has aged very well. This is as good as it gets as far as aging for pretty boys gets.
Johnny Depp on the other hand almost reminds me of Marilyn Manson who also got fat and ugly. The only kind of men who age even worse than pretty boys are these androgynous/sorta-feminine types. They age like women themselves, they get fat and end up looking ABSURD. Also, Johnny Depp wants to play these quirky roles, these eccentric comedy kinda roles. This is the only thing he's really good at, he didn't develop the same way diCaprio did. So he is far more depressing to look at, because now that he lost all his good looks and isn't even fit anymore, his usual shtick doesn't work anymore.
As a side note, young Johnny Depp always looked racially ambiguous - with the eyes, the hair and everything. But now he really just looks like an old white pervert or something. Like the kind of guy who commits suicide in a Bangkok ladyboy brothel.