Are games with hardcore porngraphy allowed on itch.io?
Adult content (such as porn) is allowed on itch.io as long as its legal. You should probably mark it as NSFW though. (This is done via the edit game page under "Metadata.")
If you're planning on charging money for it, things get a bit more complicated as some payment providers are less-than-welcoming of earning money from such content. While itch.io allows you to put this kind of stuff up no questions asked (assuming it's legal) you should probably check the terms and conditions of whatever payment provider you intend to use if you intend on profiting from this.
More info can be found on the Creater FAQ here.
"If its legal" is a bit of a misnomer in some cases, as different countries have varying laws and guidelines for (fictional) material. A good general rule of thumb however is to strictly avoid anything involving anyone under the age of 18, or anything that may appear to be under the age of 18 (so sexy time with a 100 year old vampire child still counts as a faux pas). You may also want to avoid extremely graphic and/or violent sexual content (IE: Explicit rape, sexual torture, etc).