Honestly, that's completely fair if you don't find the gameplay engaging enough. I know this game probably won't be for everyone, given I had only so much time to work with so I didn't have time for, say, dedicated difficulty settings or other such things that could've probably helped with appealing to a wider audience.
That being said, I wonder if you might be the sort of player I built the ranking system to be for. I tried to balance the game such that most people should be able to beat the game without too much trouble, for those who just want a simple almost Mario 1-ish sidescroller to run through while getting increasingly powerful in a power trip kind of way, but that getting a high rank is quite a lot harder and requires a lot more effort. Of course, the several endings thing and short length are both to try to encourage playing it enough times to start getting a feel for it.
Being tired could factor in too, of course. We'll see I guess! Feel free to share more thoughts if you play it again and have more complicated thoughts, positive or negative, afterwards. I'm definitely curious to hear more!