Mixing different pixel sizes together looks strange, I don't recommend doing it.

The bottom of the house isn't isometric.
Some of your AA (anti-aliasing) isn't really helping to smooth the image, and instead looks like misplaces shadows. AA can also thicken lines, which isn't really appropriate for something like a corner that's facing the light. Using a lighter colour and not AAing can be more effective in those situations.

The turtle seems to be floating above the grass. I think you should get rid of the outline on the turtle, and have some of the grass overlap the turtle, so that it looks like it's in the grass.
I also think you overdid the AA on the turtle. With this small size, you barely need AA at all.

Here's an edit removing unnecessary AA and some unnecessary colours:

I also made the grass overlap the turtle, made the grass gradient more grassy instead of plain, and tweaked the outline colours to give a better sense of lighting. Coloured outlines aren't a good option if you don't know what the background of your sprite might be, but they work well in situations like this where there is a fixed background.

By the way, I recommend not working on a white background, as that can throw your values off, since everything looks dark compared to white. Try working with a midtone grey. That'll also make it harder to ignore unfinished parts of the background :]

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