I must apologize for the gratuitous amounts of pink in this post - I in no way assume that all Sweet Lolitas love pink, so feel free to swap out pink for your color of choice when it comes to decor. These are just guidelines, really. Mere possibilities!
If you're lucky enough to live somewhere that allows you paint your walls, go to town in the bathroom. This is the place you're going to bathe in and likely apply your makeup or do your hair. Make it cute!~ Paint the walls in a light tint of your favorite color or apply a really cute wallpaper. Baring that, a cute bathroom rug and a ruffly shower curtain can go a long way.
There are a lot of directions you can go with bedroom decor, but stick to a theme. Be it butterflies, teddy bears, historical cuteness or whatever. Gauzy curtains give the room a dreamy feel. A vanity table or tea table can be a cute place to display some of your favorite lolita accessories or stuffed animals.
A lolita's wardrobe is very important, so making sure it looks adorable is also important. Removing closet doors and instead hanging up curtains to shield your pretty dresses is a great idea. You can also get curtains that match your window drapes and pull the whole look together.
You might think cute-ifying a kitchen would be difficult but it's really not. It all boils down to your appliances really. And you don't have to match here - but brightly colored coffee makers and toasters will really liven the place up AND match your cupcake coffee mugs! White furniture looks really good with a pastel table cloth, but you can always paint your table and chairs for a more personal look. Don't forget to showcase your cutest apron!
Living rooms are far more versatile for the Lolita. If you adore the Rococo period, why not invest in some really opulent furniture. You can add some ruffly throw pillows and a historically themed painting to keep it simple OR go all out with as many colors as you want. You can even hang your favorite JSKs or OPs on the walls for unique decor.
The key points are obviously cute, girly, and sweet. Colors are up to you, along with any historical preferences. I hope this helped!