I've seen so many 3d art such as SFM renders getting declined at this booru, and I've read one user expressing their disappointment on how their renders got declined because their posts "did not meet minimum quality standards."

Given that some artists are still learning 3d and some renders are far from Pixar level quality, are there any specifics and criteria on how 3d art that meets the "minimum quality standards" to be accepted in this booru?

https://e621.net/wiki_pages/uploading_guidelines#quality

I don't see specifics about 3D art. But anyone can just take a look at the 3d tag and see what gets approved and compare their work, you don't need to be "Pixar" to be descent at this media. Also that you lose nothing by uploading your work, the worst can happen is getting your submission deleted, which is why e6 shouldn't be used as a personal art gallery.

You can find it in "what does the deletion reason mean?" and "does not meet minimum quality standards."
topic #23578

In short, natural posing and good quality, matching textures seem to be the minimum requirements.

I'll be honest, 3d stuff is given a really insane level of leniency here already to the point where I wouldn't mind more direct clarification just so a greater level of scrutiny is held to the medium.