Is that a racial slur (i.e., Asian), or about Down syndrome? The latter was used in the UK many years ago, I've not heard it for 25 years (?), so "mongo" wouldn't raise an eyebrow here now.
It's kinda both. Mongoloid here used to refer to people with Downs, by comparing them to the (non existent) Asian race of Mongoloids due to both often having eyes with epicanthic fold. Of course I didn't know this when using it as a kid back in the days, and I've luckily not heard it used for ages, but I think all of my peers would remember it if used on a t-shirt.