i've used opengl core profile for a while now and i was wondering if it was possible to create a "decent" application (like a game/game engine) with good graphics. I have a machine which can run only OpenGl 2.1(raspberry pi3), i already know the advantages of core profile, but is Opengl 2.x still usable for today's standards?
First of all, you are very confused.
There is OpenGL ES 2.0, and there is desktop OpenGL 2.0 and 2.1. These are very different things, which run on entirely different platforms. ES runs on primarily mobile hardware, while desktop GL runs primarily on desktop hardware.
The core/compatibility distinction only exists for desktop GL, not OpenGL ES.
Raspberry Pi, of any version, does not support desktop OpenGL of any version. It only supports OpenGL ES 2.0.
As for what you can do with ES 2.0, a lot. You'll find Raspberry Pi's GPU to be limiting from a performance point of view, but you will have access to basic shader functionality. You can do a lot with that.