Cool Undertale/Deltarune easter egg!
Alright so I'm not sure how well known this connection is, but I only found out about this today :D I know people have already pointed out the connection between the FIGHT button and the significance of the LEFT EYE.
For those who are unaware, there is a pattern between facial expression and the FIGHT button in Undertale's menu screen: in both Undyne's fight as 'the true hero' and Sans' fight (subtextually hinted to be centered around justice also) both of the characters have their LEFT EYE activated.
While from the player's view it appears to be the right eye, it's actually flipped from the monster's perspective, proving it would be their LEFT EYE that is activated.
This is the same position we find the FIGHT button in the menu. Interestingly, the FIGHT screen also resembles an eye itself. This further cements the idea that LEFT means violence, and RIGHT means pacifism.
Some eagle-eyed viewers have also noticed that unlike in other routes, Sans favours winking with his LEFT eye instead of his right. All of these connections can be found within the No Mercy Route.
And in Deltarune, the eye-walls seem to resemble the 'FIGHT glare' as well!
The eye shape is similar, and there's even a line in the centre of the shape, which mirrors the mechanics of the ATTACK minigame; in order to maximise damage, the player needs to strike precisely in the CENTRE of the EYE.
So yeah! Probably not the first person to find this connection, but it's really cool! And nice to find some symbolism from this otherwise very cryptic setting in Deltarune.
Also, this connection is more of a speculation, and a pretty tenuous connection at that, but these puzzle-eyes kind of remind me of Chara's 'creepy face' from the No Mercy Route? Perhaps the 'melty eyes' is just a side effect of having too much 'LOVE' in your system. Or more likely, the 'melting' is a reference to Gaster's face.
Anyway, thanks for reading!