Animation of avatars is usually based on rotation, and if the bone direction on your avatar rig is incorrect they will animate the wrong way. Most applications allow you to rotate the bones so the normals are facing the right way.
In blender you can adjust the 'Roll' of a bone easily in edit mode.
Adjusting the bone angle using Roll in Blender.
My Avatar polycount is too high
Polycount is an important element for performance of a space. So getting the polycount down is a good skill to have. If you have a high polygon character you could use retopology to 'trace' the object with a low polygon object, and bake that high polygon information onto the low polygon using textures (like normal maps).
My avatar material count is too high
Material count is an important element for performance of a space, as each material is a drawcall. So reducing materials as much as possible is important. There are some plugins for 3D Creation tools that allow you to compress materials into one material.
You could also bake multiple materials into one using blender, substance painter etc.
My Avatar texture filesize is too large
These should be resized in the image editing application before exporting your VRM from Blender, Unity etc. We recommend having no more than 1024x1024 textures, less if possible.
My Avatar doesn't have any metadata/details
Depending on how you create your VRM, there are methods to create the Metadata on your VRM. UniVRM forces you to add this information when exporting it. VRM for Blender does not. However this can be added in the VRM tab.