I built a PC for the very first time however I'm very unhappy with what I achieved with cable management. I spent hours trying to make it look 'better' but I can't seem to do anything with this.
How bad is this? What should I do to make it look much better?
I look at the back and it looks awful lol. Also I have one problem, this cable seems to be kind of slanted and it's impossible to keep it straight due to the stiffness of the wire, should I be worried?
Use a hairdryer. Carefully. Most of the stiffness in that usb isn't the internal wiring, but the external jacket. Pull it out and gently warm it up with a hairdryer and it'll soften the jacket enough to allow it to do what you want.
Don't worry about being perfect in the rear. Not unless the back panel is also TG. As long as the cabling is decent and not pinched or suffering extreme sharp bends, it's fine. Straight is nice, but crooked works just as well.
For the front, that's an aesthetic thing. As long as wires are out of airflow or moving part or pinch/rub it doesn't matter to the pc how they are run. That's upto you to decide if you like it or not or can live with it or not. Some ppl would be more than pleased with your efforts so far, some are too OCD and wouldn't be happy with less than perfection.
You've done a lot more and gone farther than most, 👍 for that, but where you stop is only something you can say. Looks very good from where I sit.