So I came up with two different coords for that version of Milky Planet. They're both very simple outfits, but represent ways that I think would be somewhat effective at toning down such a super sweet print.
This first coord, while still definitely a sweet lolita outfit, tones down the dress by minimising details and introducing a third darker colour. The slight gothic-ness of the boots and the pointed tips of the sailor blouse give it a slightly more edgy vibe. Introducing navy [though not too much of it] brings some restraint to the bright colours of the dress. I'd probably wear this outfit with either a full size hat or a very simple hairstyle, like braids with bows at the end, to complete the look without adding any more stereotypical sweet lolita elements like a headeating bow. But I couldn't be bothered scouring the internet for the perfect headgear so please use your imaginations for that bit ^__^
Again, this coord introduces a new colour to complement and tone down the print. It still has a lot of cute detailing - the blouse with its crossed neck ribbons and lacy cuffs, the soft beret, the pom-poms on the shoes - but the colour here is what really grounds this outfit and changes it from being a colour explosion to being a gently framed colourful piece.
You'll notice with both outfits that I've used pantyhose instead of socks and have kept away from typically sweet accessories like bows and anything particularly frilly or lacy. With a print this in-your-face I think it's best to use more understated accessories if you want to make it less conventionally "sweet lolita". So though both these outfits would still fall under the sweet substyle, they are definitely not your usual sweet lolita outfit.
I actually had a lot of fun putting these outfits together, so if you happen to have a dress you'd like to see in a certain type of coord, please comment [like I always say with my hypothetical wardrobes] and let me know ^__^