To be fair, SR7 isn't an "amateur" artist. He's been drawing rule34 content for a very, very long time, pretty sure it's well over a decade at this point. What we're seeing isn't degradation of skill, or SR7 using his own artwork as reference causing proportion issues. Is he probably using his old artwork to provide a basis for his current stuff? Yes. Is it why his new stuff looks bad? No. The new Catwoman is a good example of this as we have no previous sexual content to go by and her previous design was essentially a slightly bustier take on the BTAS design.
The difference between the new and the old is too severe for it to simply be degradation of style. This is a conscious shift in artistic style to facilitate faster creation of content and to pander to the support base he and his partner have cultivated. What we're seeing is a decision to cut away as much variety as possible, where the only body types are busty, bustier, and even bustier with muscles. Even if we ignore the fact that he's copy-pasting characters together resulting in deformed monstrosities, or that Lex has a telescopic dick that in some scenes looks only slightly larger than average and in others is like a foot and a half long and thicker than a can of coke, or that he's stopped maintaining a consistent design across content, the fact of the matter is that he's reduced his art style of a "one size fits all" take on the characters where the resemblance to the DCAU originals ceases outside of their faces. This is too specific and targeted of a shift for it to actually be degradation.