"But Biden, so far, has been impregnable. The voice is too bland and devoid of obvious quirks..."

"... and beyond the occasional 'C’mon, man,' his conversational manner too muted and self-effacing, to give the parodists much to work with. Trump supporters and Fox News pundits would undoubtedly attribute this to the media’s liberal bias. And to be sure, Trump was viewed by the (mostly liberal) satirists not just as an irresistible comic target but also as a dire threat to the nation. Biden’s pleasantly boring presidency has been a welcome return to normality — but hardly great material for parody."

I picked that quote from WaPo's "Comedians are struggling to parody Biden/Let’s hope this doesn’t last" because the word "impregnable" caught my eye. 

I realize it's not meant as a pregnancy metaphor and we're supposed to think more in terms of a fortress. It's not normal to say that sterile women are "impregnable." The original meaning of the word is (from the OED): "Of a fortress or stronghold: That cannot be taken by arms; incapable of being reduced by force; capable of holding out against all attacks." Then there's the figurative meaning: "That cannot be overcome or vanquished; invincible, unconquerable, proof against attack." 

So the assertion here is that Biden is so neutral and featureless that an impersonator has nowhere to go. He's like a giant wall with no footholds. What can you do?! He's normal, so pleasantly boring. This is why I don't watch "Saturday Night Live" anymore. They're too lazy! Trump was ridiculously easy. I guess they loved not being challenged.

How about observing Biden and finding what is distinct and capable of mockery? I think the problem is really that they don't want to expose his flaws, that they're committed to the idea that he's normal and pleasant. But they ought to see this as a fantastic opportunity. The best presidential impersonation in this history of "SNL" was Dana Carvey's George H.W. Bush, and H.W. had the same problem of superficial ordinariness.