There's a time and a place

Addl wisdom: some people make jokes when they're uncomfortable, as a defense mechanism. If you have a player or players constantly trying to lighten the mood, consider that the atmosphere you're trying to create may not be a good fit for your party, and/or parts of your party may not be a good fit for your game.
- Curse of Strahd be like...

thankfully i have no idea how to make a good atmosphere in a game so that's never a problem

You're not the asshole, you're playing with the wrong group. Find people that want to play the same style of game as you, don't play with people just because they're who you're familiar with. Recipe for a bad time.

Sometimes when you're new and an atmosphere starts forming you can find yourself making jokes to avoid 'falling in' to it as a reflex - because some part of you is trying to reassert control over the emotions that were getting touched by it.
This was my experience and I've seen it happen to others. Just try to recognize it if that's happening to you and try to embrace it instead of pushing it away.
Or maybe your group likes to make goofy jokes the whole time, thats a different experience but also valid.
My best experiences have been a split, lots of little jokes then suddenly we all get really into something without expecting it and they drop away for a bit and re emerge later.