I’ve been there a few times when my daughter was little, which was about ten years ago. I always hated it because I felt like everything there was dirty and greasy from the greasy foods and the kids whose hands are covered in grease from the pizza. They wipe these things down with greasy rags, and the carpeted floor has probably absorbed gallons of soda over the years of spills. To me, it was totally gross. It is noisy like an arcade, with lights and moving ride things. Kids love it, as others have said. I really don’t remember if the food (pizza) was gross or edible but it was probably some frozen stuff, not “real” but not awful.
Like @Dutchess_III says, your niece wants you to go so you suck it up and go. Just go for an hour, put a smile on your face, and she will be so happy. She will have her friends there too, and she’ll be thrilled. II you’re lucky there will be a good cake. This is the sacrifice we make as adults, for the kids in the family and kids that we’re friends with.