"Watching @americanidol for the first time in a decade. You never know what you've got til it's gone.... Well... now I know why the ratings are down," he tweeted before adding a 'snoring' emoticon.
"Oh good Lord... those boring a** responses from the judges!? Where is Simon when you need him!?... It's VERY clear now that @SimonCowell was the reason @AmericanIdol was a hit. I've watched root canals more entertaining than these judges."
He later returned to the site to explain his rant, adding, "When your favorite team that you've cheered for for years happens to play a (bad) game, you're absolutely allowed to be upset about it.
"We're all more critical of the things we care about most... When you really love something you only want the best for it, and you have high expectations from those you know are capable of it."
Former American Idol winners Kelly Clarkson and Carrie Underwood also tuned into the show, but only had positive feedback on social media.
Pregnant Kelly wrote, "Watching @AmericanIdol & totally crying like a pregnant girl every time some1 (someone) makes it ha! Last season, so crazy! Thanks 4 (for) my start Idol."
Carrie added, "Been walking down memory lane a lot lately... hard to believe it's @AmericanIdol 's last season! Thank you for making my dreams come true!... Wow. I might have found a season favorite already! What a beautiful voice..."
Simon is set to appear on American Idol with original judges Paula Abdul and Randy Jackson to celebrate the final series.