If that were true, then Labour has a bigger anti-semitism problem than is made out. Fortunately, it's mostly Labour MPs and Labour supporter groups that are demanding change from their own party. There's no smears: the mural was real, it was removed for appearing anti-semitic (the reasons explained above in that well known Tory rag "The Guardian"), Corbyn did comment on it in apparent support for the artist. A Jewish Labour MP brought it to the public's attention. Labour-supporting Jewish groups protested against him, Corbyn apologised. But Jewdas publicly criticised the protest. And Corbyn's first interaction with the Jewish community following his apology was with the dissenting Jewdas rather than engaging with those who had the problem. Which has understandably further antagonised those who were looking to build bridges.
The actual Tories (ie the Conservatives, not just anyone with moderate left to right wing views ) have no interest in getting involved, they've got their own problems and are quite happy for Labour to self-destruct. But none of that seems to register with the Corbynista apologists who seem to just want to discredit any source of criticism rather than acknowledge the problem and stop it from causing real damage.
And the thing is, if it was say Theresa May coming under fire for commenting on the mural, I'm sure no one would think saying the artist just couldn't draw very well was a reasonable explanantion.