Kevin Brennan -- a former 'SNL' writer and comedian -- is making light of Matthew Perry's passing in a real ugly way ... which may speak to why he isn't more successful in showbiz.

Kevin -- who had a 2-year stint writing for the famed sketch comedy show way back when, and who was once a rising comic -- took to Twitter this weekend after news of Perry's death broke ... and posted an incredibly insensitive message mocking the late 'Friends' actor.

He wrote, "DROWNED IN A HOT TUB. HAHAHAHA" ... this while linking back to our story.

The tweet wasn't viewed by very many people -- KB only has about 5,900 followers or so anyway -- but it certainly left a bad taste in the mouths of folks who did see it ... with many people calling him out in the comments and bashing him for making a joke out of it.

While Kevin isn't nearly as relevant as he was in the '90s/early 2000s ... he still has a famous connection to mainstream Hollywood through his younger brother Neal Brennan -- from whom he's estranged. Neal, of course, is the co-creator/writer of 'Chappelle's Show.'

Neal and Kevin had a falling out years ago -- and the younger sibling went on to forge a solid path for himself. Neal's written for tons of shows, and even has his own Netflix comedy special now ... whereas Kevin hosts a pretty obscure podcast, and does gigs here and there.

It bears repeating ... Neal is NOT close to Kevin -- and based on interviews NB has done of late, where he was asked about his relationship with Kevin ... you can tell it's a sensitive topic. Based on this reaction to Perry alone, you'll see their personalities are very different.

So, of course, Kevin's crude remarks here are in no way a reflection of Neal -- who, as far as we can tell, minds his business and focuses on his work ... and who doesn't seem to have talked about Perry or 'Friends,' for that matter, very much at all publicly.

Kevin appears to still fancy himself standup-adjacent -- regularly commenting on mainstream comedians that Neal undoubtedly rubs shoulders with ... although, what he pulled here is far from a laughing matter. Neal doesn't seem to have reacted to his brother's post.