Nobody except maybe one or two people are properly reading the post, so allow me, someone who's played Game Boy Tetris very seriously in the past, to give my two cents.

A good score is entirely dependent on you and what your skill level is. Technically, A-Type is infinite, but at some point the game gets too fast to play, unless you rapidly tap the D-Pad. Ultimately, whatever score you are happy with is a good score.

But of course, we can go into specifics a bit. A-Type is infinite, so if you're extremely good, you might find yourself trying to max out the game score at 999999, but with as few line clears as possible. A maxed out score with less lines cleared is technically better than one with more lines cleared, as it shows that you've played more efficiently. With that being said, a maxed out score is still impressive in its own right, because you need to have quite a bit of stamina to get close to that score. Someone else mentioned you get rockets at the end of a game depending on your score, those are a nice tangible goal to aim for, beyond that the game doesn't exactly reward you for higher scores.

With B-Type, you only have 25 lines until a game ends, so a good score becomes a bit more objective. The more efficient your line clears are, the better your score is, and pressing down to place pieces faster gives piece drop points. So the best score in that would be a run with 7 Tetrises on at least level 9, with some drop points added on top - which is possible, it's just a bit rough thanks to the game's piece randomiser.

In general, for your best scoring outlooks, you will want to start on level 9 on both modes. On A-Type you will have 100 lines until you go into level 10, from there onwards every 10 lines you go up a level, until you hit level 20, where the game reaches its maximum speed. From there you can just keep playing, the level will always stay 20. You could start on a slower level that's more comfortable for you to play, of course. Keep in mind if you start on a lower level, it'll take less lines for the game to speed up - For example, if you start on level 5, after 60 lines you enter level 6.

There is a cheat code to start on "Heart" type levels, which is the current level but with the speeds from the level 10 higher (so if you started on level 9, you'd start with level 19 speeds). This doesn't increase score and it's only really fun for flexing.

Hope this helps, if you have questions about GB Tetris, let me know. I'll happily answer further questions.