How to cover up ugly large vent cover

That vent is necessary. Just paint it the same color as your wall.

Make a feature out of it. Light weight picture frames. Of family or whatever. If frames are light enough you can use straight pins to hang them without worry to damage vent inside.

Here are some ideas to help disguise it:

https://www.pinterest.com/explore/return-air-vent/?lp=true

https://www.apartmenttherapy.com/cold-air-return-covers-and-other-vent-camouflage-ideas-248645

The vent is for ventilation. It would not be wise to cover it completely unless the system has been redone and it no longer serves any purpose whatsover. As Karen said, if it is painted the same as your adjacent wall or ceiling cover it will be less noticeable. Good Luck and be safe.

Don't block a vent. You can paint it to blend into the surroundings.