Tom Brokaw -- Lindsay Lohan RUINED White House Correspondents' Dinner

Lindsay Lohan DESTROYED any shred of dignity the annual White House Correspondents' Dinner had left ... and now it's nothing more than a joke ... this according to Tom Brokaw.

The legendary newsman blasted the dinner during an interview with Politico -- saying the standards for the once-respected event have fallen so low, his daughter's junior prom has more dignity.

FYI -- Lohan was a guest of Fox News’ Greta Van Susteren at last year's event ... where booze flows freely and Beltway nerds routinely bring show-offy stars as plus-ones.

Brokaw says he's so pissed at the downfall of the event, he won't ever attend again.

However, there was one wild and crazy celebrity appearance that he actually enjoyed -- wait'll ya hear who it is.