Recognizing fallacies is crucial for evaluating the validity of arguments and making sound judgments.
The act itself is morally unacceptable and grotesque no matter what age or if it was done in public rather than private. So, utilizing Trump to convey the message in such a way simply questions Trump's own moral judgement on what he may deem acceptable from an already grotesque act.
Anybody who has seen circulating photos of Trump with his daughter in inappropriate poses, and heard how Trump has made inappropriate statements and phrases regarding his relationship with his daughter...could clearly see Trump deeming the act acceptable as long as daughter is of age and done openly in public.
And no, I don't defend grotesque acts of Biden.