Careless is what I take to mean acting without any care.
Carefree is the word you're looking for in the title.

This is good.

"Clouds form above us
white as the snow in winter"
Good opening, a bit of a cliched comparison to the snow in winter, but it's still a good opening. Good imagery.

"beautiful as always"
In nature poems, everything is described as 'beautiful'. Maybe read a few more of them on this site and be critical of them, see the reoccuring themes in them and try to write something a little more original.
Oh and I did an activity in english a few years back called "show don't tell" where with writing you need to show the reader how it is beautiful rather than tell them that it is beautiful. I know you've already "shown" the reader how it's beautiful in the beginning, but perhaps focus in again on showing the beauty in this line.
Here's hoping that made sense.

"we're laying on soft green grass"
With longer nature poems it's okay to cross themes a bit but you've gone through three in the poem, 1. coulds 2. grass 3. carefree-ness(?) which is too many for this poem. Though maybe if in future you changed the third line as I suggested previously it would work better for you. I like the imagery again created in this. Don't we all love the feel of soft grass?

"without a care in the world."
The ending is a little overused, but not too much. Still effective as it brings the poem together.

Sorry that this comment is a bit all over the place. I did like this and nature poems are harder to write than most others, I find. It's still a 5/5 poem.

jess ~

"Clouds form above us
white as the snow in winter"
`Simply beautiful. Such a great comparison [simile] to start off the poem. Simply said yet, its so easy to picture. :]

"beautiful as always"
`I think you have the syllable count off here. I'm getting 6,and its supposed to be 5.
beau/ti/ful as al/ways
Unless always is just one syllable, but I'm pretty sure its two. You may wanna check that.

"we're laying on soft green grass
without a care in the world."
`Good syllable count, it matches the recommended syllable. Well done. So beautifully described.

Overall, a great Tanka, just that one line with six syllables was the only thing I saw wrong. Otherwise.. well done. (; 5/5.