It is! You don't need to create a functional ROM for the gameboy - you just have to stick to the simple rules of making it 160x144 and four colours. There are a lot of people who have used GBJam as their first jam - I'm one of them!
Entered my very first gbjam last year and had a blast. It was just like making a regular game, except you're limited to 160x144 resolution and any 4 colors of your choosing. It was a very fun challenge. Gameboy games are relatively simple in terms of gameplay because of the small screen size, so it's perfect for a beginner imho. Highly recommended!
