There is nothing wrong in being cheerful and funny around people, even if your true personality and thoughts are quieter and deeper that that. I think you are wanting to tone this down though.

The only way to break a behavioural habit is to 'act' another way and it will soon become not acting but more yourself as you gather courage to be yourself in front of others. So I suggest you try replacing 'idiotic' with 'cheeky' - its just...better, and also leaves room to throw in a deep and meaningful to counter it - and make people think.
Think twice before you say anything - did you really want that to come out like that..? Even at my age I still have to bite my tongue sometimes to not appear silly.

I too am quite 'deep' with my thoughts, poetic, prone to melancholy and a hopeless romantic. But I am also incorrigibly cheeky, fun and full of life which draws people. Then with a chosen few, we can sit down and have a proper conversation about things that matter.

Good luck and never, never, never give up. If you really want to achieve something, you just will.

You don't have to be serious all the time, but I understand if you despair at not being able to be serious any of the time. That takes courage