You might want to look up some reference images of shoulders and necks. The line of a person's shoulders when their arms are at their sides is not horizontal (unless they're wearing shoulderpads) and the neck is not a pair of perfect vertical lines. Looking at renderings of the muscular and skeletal systems is helpful! (the muscles at the front of the neck form a V-shape from the sides of the jaw to the centre of the clavicle; the muscle covering the back of the neck and shoulders causes the shoulders to have a sloped appearance. here
is an example.) The place where the neck is meeting the shoulders forms a right angle in your drawing, particularly noticeable on NOFX-guy; on real people this is more of a curve.
The other thing I'm noticing is that you are using a very saturated palette. There is nothing wrong with saturated colours, but your palette feels a bit... disjointed? Colours are altered by lighting, which unifies them into an overall image that looks like the colours belong together. This
is a great tutorial on colour use. c:
You have a very cute and cartoony style, especially the kitties!