Joe was celebrating his born day Monday, and it looks like his staff lit up the festivities with a crap ton of candles on a meager-sized pastry for POTUS -- something the Commander in Chief appeared to appreciate.

As you can see, the cake looks to be set completely ablaze ... and people didn't hold back in making cracks and memes on the Internet ... joking about everything from the White House at risk of getting burned down, to comparing it to the state of the country under Joe's watch.

As funny as some of these comparisons were -- several of which were celeb-inspired, in fact -- Joe himself had a joke to top it off, writing ... "Thanks for the birthday well-wishes today, everyone. Turns out on your 146th birthday, you run out of space for candles!"

It's an interesting way to embrace his age -- namely, tongue-in-cheek humor -- as the Prez being a super senior has fallen under intense scrutiny ... especially as 2024 approaches.

For instance, just the other day, he mixed up Taylor Swift and Britney Spears. Obviously, not a huge problem ... but c'mon, man!!!

And yet, he's taking 81 in stride -- rolling with the punches and jabs. Time will tell if it's an effective strategy ahead of the next presidential election ... right now, his poll numbers stink.