There is a magnetic top vent filter that comes with the case but it covers the top rear exhaust fan so I don’t really want to put it on.
Yes, I know the pc is dirty, but I’m getting a new PSU after Christmas so I’ll clean it then.
Safe to put on, and I would. It will prevent dust from collecting via gravity, and it will keep dust from getting sucked in during normal use.
It may not look like it, but even if your main intake is at the front blowing horizontally, some air is inevitably being pulled in from the top by your CPU cooler and the exhaust fans.
It won’t hamper exhaust capabilities to any significant degree.
i personally leave the dust cover on the top unless I’m playing something that will heat the pc up a bit more than normal, then I take it off to help with cooling. I have birds, so their dust and feathers seem to migrate to my pc, so normally I just leave it on at all times.