[Showcase] My upscaled sprites

For Bulbasaur you need to tweak its front left leg to be as thick as the right one. In the hind leg the nails need to be moved 1 or 2 pixels to the right to match the perspective The bulb just needs some adjustment on the right (your right) curve lines to look less "pointy". Its right eye needs to be a bit thinner to be on par with the perspective and the shape of the head, and the "wrinkles" on top of the eyes should follow the shape of the eyes. The ears need to be a little bit shorter and the mouth a bit wider to your left.

Next is Ivysaur. The flower bud is good shape-wise, only missing the little details on the top. Overall shape of the head is ok. Same problem than Bulbasaur with its left eye here and the eye wrinkles. Its left fang should be 1 pixel lower, as well as the corresponding line of the mouth, and 3 pixels to your right. The center leaf is a bit messy and needs to be redone to look more consistent to the original. The frog-like feet are an artistic decission of yours so... I don't have more to say.

Venusaur is almost perfect in proportions and shapes, the only detail is that you misunderstood the seed on top of its flower as being part of that crown-like stuff; and the positioning of the nails in its left front leg is a bit off.

Next comes Charmander. Its head is ok. Its right eye doesn't need an outline, look at the original for reference, there the eyes look more natural and intergred to the face. And the fang maybe needs to be 3 pixels forming a triangle, but I'm not really sure. The top part of the belly needs to be thinner than the bottom. The tail also needs to be thinner, look that in the original the tail is not much thicker than the left arm; and I like what you did with the flame. The arms and legs are fine.

Oddish and Voltorb are good, they have some differences to the originals like Oddish's feet and Voltorb's expression but it's fine if you don't want to do do a 1:1 copy of the originals. Oddish needs a very small tweak on that line in the center on the big leaf, just make that line end in the upmost part of the leaf.

For the shading the most important thing is the lightsource and how it interacts with the body parts. Pokémon tend to have a cartoony shading but still there's some logic behind it. In official sprites, the lightsource usually comes from above, though sometimes there are other sources of light such as Charmander's flame. You have to account for differnt kind of surfaces as well. For example, Oddish has a spherical / curve body, but you gave to it a shading that is more akin to plain surfaces, the same happens with the bulb of Bulbasaur.
In the eyes, the irises will always have their own shading independent from the lightsource to give them more depth and personality. The sclera (white part) will be shaded according to the shadow projected by the eyebrows, always caring to not make the eye look hollow. That problem is present in your Ivysaur. Last but not least are the outlines. Black outlines tend to accompany the darkest shades of colors, so when outlining a light color use a darker shade of that color instead of a black outline.