The tree feels very much like a symbol for a tree rather than a tree. That could look good in a game world built entirely of such symbolic representations, but compared to the more rendered grass and dirt, it just looks like a green blob on a brown stick.

Try to get away from such simple symmetric shapes like cylinders (trunk) and speres (crown), and use more complex shapes with gaps, asymmetry, bits jutting out, etc. It's easiest to just doodle some shapes instead of trying to think hard about it. Do a bunch of sketches, something like this:

(These are more side-view-esque, RPG view would look a bit different, but you get the idea.)

I'd also like to see a mock-up with an actual level layout.
I think the most important thing right now is to make the platforms stand out and be immediately visible as something a player can jump/stand on. After that, you can worry about prettying up the background and conveying a sense of place with the art. The grass is a good start since it's bright and visible, but it's on only one platform.

I think once you also change the remaining transitions it'll look better. Right now most of your tiles still have a very straight transition. You're also under-utilizing your corners - arrange your tiles in more exotic, staggered shapes instead of rectangles and straight lines, which require more use of your corner tiles. Corner tiles are very important to being able to build interesting shapes, and you haven't given them better shapes yet.

Try making some mock-ups with just solid colours (green and brown) to play with the shapes, do you don't spend time pixelling detail in a tile that might not work well.

If you're going for more naturalistic transitions instead of a blocky, clearly-tiled look, you shouldn't use such straight transitions. Have bits going in and out, and don't be afraid to let things go all the way to the opposite edge sometimes. If you just have a straight line boundary in your transition tiles, that's only slightly better than not using transition tiles at all.

Also, try not to repeat parts of the other tiles in your transition tiles, as that only undoes the purpose of using multiple tiles. Take the time to make new grass/dirt bits that tile with the others. It's not that much more work, but it looks way better than repeating elements.

Here is a pixel-over where I only changed your side transition tiles, to give you an idea of what you can do:

It's two tiles total (plus horizontally flipped versions), same as yours.

If you're clever about the exact shapes and transitions between the tiles, you could even avoid having dedicated corner tiles and instead have "side" tiles that can be used for corners too. Making multipurpose tiles like that can make it easier to make varied, natural-looking maps with the same number of tiles, or even fewer tiles. Here is a somewhat extreme example using 16 8x8 tiles (zoomed in and with a grid on the example usage), where almost every transition tile can be used as a corner or side, and a few can even be used for both horizontal and vertical transitions or as "islands". With 32x32 and with transitions between more distinctly coloured terrains it'll be much harder to achieve that degree of flexibility, but you can still make something much more flexible than what you have now.