President Biden's lighting up the Internet with his fiery birthday cake poking fun at his age -- but that won't get him over the electoral hump with the young'ns ... so says a seasoned journo.

Technology reporter Taylor Lorenz -- who covers all things digital for the Washington Post -- weighed in on Joe getting a little cheeky this week to celebrate his 81st birthday ... with a jokey post that ended up fueling endless memes online.

Everybody, it seems, made cracks ... some were light-hearted, others were a little more serious -- but according to Taylor, none of it really helps the Prez in the long run for 2024.

As she put it on Tuesday's "TMZ Live," Biden and his staff are missing the mark on how to connect with Gen Z -- mistakenly thinking that viral content and memes alone will be enough to get a full base of potential voters to pull the lever for him come next November.

Lorenz says that's not gonna do it, unfortunately -- and that Joe needs to connect with young people on a more fundamental, serious level ... through policy that helps their lives.

There are a number of issues she thinks JB's out of touch with when it comes to this specific demographic -- everything from the economy to housing and healthcare ... and even the war between Israel and Hamas. On that topic, too, she says Joe's not reflecting their view.

Of course, there are also the Biden stumbles and fumbles that happen all too often -- including recently mixing up Britney Spears and Taylor Swift -- that Lorenz says aren't serving him well.

Bottom line ... TL thinks Joe's gotta sharpen up and engage 20-somethings with substance, if he wants to rely on their support at the polls ... 'cause right now, the gimmicks aren't cutting it.