Why does Hillary Clinton tend to under-perform expectations?

[Landslide Lyndon]

She still under-performed expectations though she only won 55-43 against a relatively weak challenger while Gore won NY 60-35. That was the only competitive election she has won in her entire career.

That's revisionist history. Lazio was pumped up as a strong challenger and the race was characterized as a barnburner that would go down to the wire.

Yeah, you can argue the media was being dumb and hyping it without any real evidence it was going to be close, but from what I've read, my impression is that this was covered as though it were highly competitive.  It's still not a very impressive victory though, NY being NY

New York reelected two years later a Republican governor in a landslide. Things have changed a lot since 2000.

[Landslide Lyndon]

Carl McCall was a joke candidate.  The state Comptroller, but a joke candidate all the same and one who received little meaningful support from his party beyond the bare minimum.  McCall also got hit with a major scandal in mid-October 2002.

Do you want me to mention how many joke Republican candidates have beaten much stronger Democratic challengers only because of the partisan lean of their districts/states in recent years?
The point is New York wasn't as Democratic in 2000 as it is now. Having a D next to your name wasn't enough, you had to actually run a campaign.

[Landslide Lyndon]

Hillary's performance in General Elections has always been solid. She solidly won (Atlas says 55.27 to 43.01, which is over 12 points) what was supposed to be a competitive race in 2000, in a state that didn't have two Democrats in the Senate consistently until 1998. In 2006, she won a crushing 36-point victory, carrying all but four counties in conservative upstate.

The 12 point victory doesn't look all too impressive when Gore won the state by 25 points. It would be impressive if it were a midterm and not a Presidential year in an open seat.

Well, it is impressive if you accept Berniebots' narrative that she is a two-faced conserva-Dem bitch that nobody likes.