For output, simpler letters are easier to read. That's why we still type and print with mainly lowercase letters, as in books, newspapers, and online text. Furthermore, all-caps style is frowned upon unless it's for emphasis, like titles and yelling. If upper and lower case are identical, then there's no point to making a separate set of letters. Many writing systems don't have lowercase and that's fine. But if you do have them, they should make a visible difference. Most importantly, it should make reading easier.
Maybe that particular font style needs some training to read comfortably. In particular, it's hard to differentiate tones 1 and 5. The other tones are fine. Maybe for tone 1, have it extend above the letter and a thin line across the top. (Like the Latin T.) Or since tone 5 is essentially toneless or neutral, you could omit the tone mark altogether.
If you haven't considered additional tones, that's fine. Not all scripts have to be adapted for all languages and universal utility. However, consider the IPA article on tones and the common features of tones across different languages. You might also experiment with tone marks extending to the left as well. That will give you more variables and combinations to play with. For instance, the four Mandarin tones can be denoted as such:
- Tone 1:
- Tone 2:
- Tone 3:
- Tone 4:
Native: English, Cantonese
Favorite Writing Systems: Chinese, Latin, Japanese, Hangeul, Flownetic