"That's not art! It's pornography!"
"That's not art! It's offensive!"
None of these things preclude something from having 'serious artistic value' if it creatively and artistically expresses a concept, idea, or emotion.
It IS art, whether you like it or not.
If art is to have an objective definition, then you don't get to arbitrarily decide what is or isn't art on the basis of whether it meets your preferences.
Pornography is art because it expresses/indulges an artistic concept.
Lolicon/shotacon is art too.
Even CSEM is technically artistic, but the difference is that it's regarded as illegal in spite of that because the rights of children to be free from abuse comes before the right to use/exploit real children as a means of sexualized expression.
Of course, drawings/cartoons/fantasy/fiction, like lolicon/shotacon, do not involve real children, so they are free from that 'real child' aspect and the restrictions/illegality that brings.