Taking inspiration from something related to space doesn't mean they are all "space skins", or that "this is a bit too much". You may as well say most of the skins in this game are high fantasy based and it's too much, see how silly that claim is?
For the record, Imma try and elaborate on how exactly those skins are not the same deal at all.
Star Guardian skin-line is meant to be a direct reference to the Magical Girl anime, namely the classic Sailor Moon, Cardcaptor Sakura and the more recently crowned Puella Magi Madoka Magica. Since all of them have something to do with constellation or the space itself in content, the space vibe it derived and put into the design, but not really to do with the artists behind Star Guardian actually looking for space themes when designing this skin line. This skin-line (it really should be called a "skin bundle") is as far as I know, just meant to be an anime themed one, to celebrate for the JP server's establishment (it's being milked now though because why not. That, and because many artists sooner or later come to realise there is actually no such thing as "anime style" just as you can't contain all movies, comics, graphic novels etc all under a single banner, and so to pull off the "anime inspired" vibe they have to always fall back on the very exclusively anime things such as Magical Girls).
Similarly, Cosmic Reaver Kassadin originated as the OCE skin, to celebrate the server's arrival, and for that purpose as well as trying to represent the otherwise very diverse region without prejudice, they went with the use of southern sky constellation as the main theme for the skin. And Riot is now building a skin-line from that one-off (which is what they do), creating the Rakan&Xayah skin as well as the Yi's skin. But hold your horses, while I am not sure what exactly Cosmic Reaver Yi's idea came from, I am pretty darn sure that Rakan&Xayah skin is actually based on Adam and Eve (especially with their pose hinting to the painting The Creation of Adam by Michelangelo, and the fact the skin is named Cosmic "Dawn" XXX), they just move the setting to space because well, why not take another approach and that's kinda it.
The Darkstar skin are based on famous fictional horror subgenre popularised by H. P. Lovecraft, the cosmic horror, where lives and planets are insignificant and meaningless before the unknowns of the void and terrible things as well as unimaginable creatures lurking in the vastness of space that we have no knowledge of nor control over. This is also why champs with Darkstar skins tend to have strange and somewhat incomprehensible forms, and fear-inducing sizes as depicted by their splash arts, and then there's Darkstar Thresh's teaser where he tried to be Cthulhu.
See, they are all very different things, they merely have some relationship with space that's all. What, do you complain when the vast majority of the skin in LoL are all based and depicted to closely resemble cultural stereotypes, historical stereotypes, or mythological stereotypes (excluding the more comical ones such as the "Kitchen" skin-line)?
If anything, the only skin-line one may argue to be maybe a bit "too much" is probably the PROJECT skin (which is also closely tied to Battlecast and the random Mecha skins here and there etc, which they are all supposedly connected. Man, now that's what we call a "surge"), but otherwise not really.
