Nice work, but keep in mind that the resulting graves are 32x32px. Therefore they do not look quite like an original vector image. In particular a higher contrast for shading and features is needed to still be noticeable. For that purpose it's also useful to try to fit the image better into the grave square to make more efficient use of the space.
A perspective view of an object is more interesting than just a side/front view where possible. Additionally, the outline is generally thicker than that on graves.
Here's a breakdown of the other prominent issues:

Atom - the features are very small, especially the lines can easily get lost.
Banana - a bit narrow, could use a different view to fit the square better.
Basketball - barely has any shadowing visible, could use a self shadow at the bottom. The black lines inside are too prominent, being the same as the outline. Generally the outline should be visibly limited to the object's outer boundary. They also could follow the curvature of the surface better.
Battery - it's basically pixel art, very different style from the usual cell shading.
Cauldron - generally usable, but at this scale need to force highlights/shadows on the cauldron's body more, and rotate it more towards the viewer so that the shape of the contents wouldn't be so thin.
Cone - looks really flat, could use perspective so that the base would be more visible. And the base it self can be quite a bit thicker.
Crown - looks overly simplistic, tbh I would recognize it as an explosion before a crown.
Domino - doesn't look very engaging (completely flat and monochromatic)
Pacman Ghost - should really be called just Ghost2. No need to go after this particular ghost style if it's going to look this flat.
Pawn - looks usable but can use higher contrast, and a wider shape.
Target - doesn't look bad, but the inner black circles can be tinted more red/blue correspondingly to not look the same as the outline. There probably should be a highlight on the red ring opposite the shadow.
Violin key - can be called just Clef. Other than that, it's barely visible at all.

Thank you. I will see what I can do.

Thanks! Here's what I have to say:

Atom: Lines are too thin, an outline might help (see Hedgewars Graphics Guideline). An animation might help as well
Cauldron: The bubbles are really small, and the shading of the cauldron is minimal. The bubbles might be better if animated.
Battery: I don't like pixel art in Hedgewars. A lightning bolt or another electricity-related logo might do the trick, not sure … But even then, I think the battery looks quite abstract.
Pacman ghost: I think it's simple and good. I'm pretty sure to pick a different name on inclusion.
Pawn: Looks nice in the full size but the details get lost in the 32×32 version
Basketball: Shading could be more pronouced. Looks rather flat. Otherwise OK.
Violin key: No outline. But I'm afraid even with outline it's too detailed for a grave. I'm not sure what others think …
Target: I reject this outright, it's too similar with the Target object. I don't want to have "decoy" graves in the game.
Crown: I agree this doesn't really look like a crown, quite abstract. Although I like the shape. Maybe this could be converted into some strange gravestone, but I don't know …
Cone: I like that one! I don't mind with this one being "flat"
Domino: It's simple but OK.
Banana: I agree that it's a bit narrow.

I think the issue with most of the graves is the detail that gets lost when scaling down to 32×32. Outlines are important because graves need to be visible against any background. E.g. the violin key would become invisible against a black background.

As for inclusion, I need to talk to the other Hedgewars developers …

Speaking of graves: I would especially appreciate the addition of a cyborg- or computer-related grave. A grave that would fit to the cyborg enemy from the campaign "A Classic Fairytale". Currently, the cyborg hedgehogs use the "ring" grave which isn't the best choice.
I don't have a good idea for a cyborg grave, however … :/

-some sort of brain in jar, similar to Mother Brain or something
-metal skull with red eyes (darkened, to look inactive), akin to Terminator
-mechanical hand
-computer monitor displaying error/R.I.P./candle/skull etc.
-like above, but hologram instead of monitor
-pile of metal junk lol

btw domino has some out-of-place pixel near the bottom.

Graves in original size for better judgement:

EDIT: Nemo thinks most of the graves feel "too flat" (not enough shading).

Hey, I agree, these graves are not good enough to be added in the game but I fear I cant help cause I am not at all an artist and dont have enough skills to draw proper graves.

Hello, I edited some of these graves following your suggestions and I created 2 new ones.

Download link: https://www.dropbox.com/s/m18248i93sssq4l/KIRAgraves.hwp?dl=1

Cone: increased contrast and improved shading;
Domino: changed colors and improved shading;
Couldron: improved shading, increased bubbles size and animated them;
Ghost2: it is old pacman ghost with improved shading and it has animated eyes and body;
Clock: a new simple and animated grave consisting in a clock with moving pointer;
Deadrobot: a new animated grave I made since Wuzzy requested it consisting in a robot head with "X" flashing eyes.