I see that this was your first game posted here! Great job finishing it and getting it done in time! This is a real achievement, a real milestone, and you're doing great. Now you just need to keep going and keep going.

Go for something much smaller next time.

First, you probably already know this, but be careful with games that require players to build them first. The more effort you ask from potential players, the less plays you'll get. (That's also why physical games have a hard time getting plays.)

So, your concept is very intriguing, but the part creation idea is a dangerous one, because it's very hard to make it balanced, interesting, user-friendly, and not go for something over-ambitious. Whenever developers talk about "infinite possibilities", everyone becomes extremely wary.

The user-created parts here have to be drawn and integrated "by hand", which is a lot of work to be done outside of the game. Since it happens out of the game, it doesn't feel like play, and can't be supported by your work as part of the game's experience - so it has no game feel to it, no support, no dedicated tools, no help. This means that it really also feels like work. Because it is. And it's also very much not user-friendly. The walls of text on the itch.io page explaining how to use the game do you a disservice.

My second gripe with it is that the parts in question can only vary along one metric: their level. There's no design choices, no trade-offs, no artistry to the conception of a specific part. Moreover, all that graphical work required is decorrelated from the resulting part's mechanical identity, and that simplistic mechanical identity itself doesn't leave room for self expression.

This is entirely natural, because this is a jam game with an intriguing but extremely ambitious concept. Here, the part designs are made to support and as part of a fighting game. Developing a fighting game takes time, so you couldn't focus on the part design. And you couldn't make the part creation very interesting, because you didn't have the time (since you were trying to make the part creation work) to build a very crunchy and interesting fighting game with lots of metrics to fiddle with, and you also didn't have the time to make the part design an element of the game in itself. You overscoped.

Be careful with overscoping, I know it's one of my main mistakes with jam games, and I know how hard it is not to do, but you have to go for very small games, especially while you're beginners. Even smaller than you think.

If you'd have wanted to make the part creation interesting, it would require the design of parts to be a fun process in and of itself. Maybe puzzly fun, or strategical and full of interesting choices, or just so full of juice it's gonna burst, or just so satisfying and eye-candy, or maybe meditative and relaxing... It needs to be part of your game. Make your premise the core of your game.

Here, the core of your game is actually the fighting, but since (I suspect) so much time was spent into making the part creation work, the combat is rough, unpolished, and falls kind of flat. 2D fighting games/platformers are also very hard to do well. And multiplayer games are dangerous to do as part of a jam, since they require two players and are thus harder to play as intended.

I think if you had the time to just take the core of your tiny fighting game, and try to make it feel really good, with some nice game feel and juice, maybe screenshakes, some sound effects, particle effects and VFX, focus of the feel of the control and make them nice to use, just make a very small combat game and make it feel good, it would already be much better. Or if you focused entirely on the part building and cut the fighting game. But just pick a side and take the time to polish it to hell and back.

Finally, you entirely forgot to include any kind of in-game info. There is no tutorial. There are no explanations, no visual reminders of the effects of the parts on the mech's stats, nothing. The UX is poor. 

My first jam game suffered terribly from much of the same problems you have here. I understand in my flesh that this is a result of overscoping. Don't worry, it gets much, much better. I think you'll find that there is pride and joy in making something extremely small very very good.

All in all, I hope making this game was a fun and interesting process for you both. Do not let this discourage you. To sum it up (and take it with a grain of salt, I'm not that good at jams, though I'm getting there slowly), next time, make the core selling point of your game the center of your design, go for something very small, invest in polish and juice, and think about User Experience and accessibility early. You're awesome and you've got this.