It looks to me like in that version (the SG-1000 version?) of Black Onyx, They wanted the font to match that thick line style, like how in the English the P, A etc. have that one thick line giving it sort of a marquee/arcade look, and they sacrificed quite a bit of readability for it. (Or system requirements necessitated it in some way - it seems like an underpowered system that wouldn’t have built-in fonts after all. The way the line width varies seems stylistic to me but there could always be some arcane reason for it)
Usually in old games I see a font more like this one in the MSX version, which is more like your example:
But I don’t know that it would be possible to reproduce that in the space required and keep the bolder strokes.
Honestly – still probably a lot more readable than the original solution on the PC-88!
Edit to add: this is going a little overboard at this point, but I flipped through the catalogue I have of old Sega games, and hilariously, I think this might be one of only two games for the SG-1000 to have hiragana in them*, the other being this Sherlock Holmes game (ロレッタの肖像) that looks like it uses the same font:
All the other games catalogued are either arcade-style games that seem like they just have assorted English words, or a mahjong game that looks like it only has katakana and a few oversized kanji.
By the time the Mk.III/Master System rolls around though, the な’s look normal and all the fonts are thin-line in the screenshots I can see, so maybe that thick-line font is just a weird quirk of Sega’s first home console.
_* there’s at least one game I missed - a shogi game! I think… I think they only implemented を, お, せ, か, and し though… and the font looks… sloppy. Albeit thin! (actually looks definitely less like a real font and more like… one asset that says to move the cursor and then press the button)
Anyway, sorry for all the irrelevant detail, apparently comparing fonts in old games is one topic to get me interested…
PPS: I found a clearer thick-lined take on な in a Game Gear game about McDonalds…
That honestly looks very similar to the SG-1000 font, but a bit clearer and more refined in some ways. It’s several years older but still published by Sega for Sega hardware so who knows, maybe there’s even a direct lineage… (I suppose probably not)
In any case I’m leaning towards the answer being “yes, the な could probably have been made more readable without sacrificing the font style, but there was probably a lot left to be refined at this point in time…”