My feeling is that if a number of Republican-oriented publications can be cited as having doubts about a Democrat politician, that might not be as reliable evidence of the actualité as one might want to have before making a judgement.
Peggy Noonan worked for Ronald Reagan and George H. W. Bush. The WSJ (a Murdoch paper) is conservative-leaning and was and possibly still is pro-Trump.
Ms Harris seems to have had a reasonable career to date, considerably better than that of Trump... But there is an assumption being made there that whereas he was able to be President with no political experience whatsoever, she would be incapable of it if (spits over shoulder) Biden were to fall ill, in spite of having a fair bit of political experience.
"Ms. Harris has never seemed especially earnest" the opinion piece asserts. Hmmm. To whom? Those who wish to portray her as frivolous and incompetent, perhaps.