Many similar services allow/require you to book an "aircraft-style sleeper seat" or similar if you're not booking a cabin berth. On some it's included in the price. Those seats tend to be in rooms that are supposed to be quiet overnight, but with many people in are probably lighter and noisier than you might like, like an oversize aeroplane cabin but with a bit more space to move around. An eye mask and good earplugs might be enough. They don't seem to list it on the website, but the picture on the amenities and facilities page shows that sort of lounge, but with a bar to one side - the question is how late it's open and how noisy it gets (they may serve quietly).
I have, on a shorter, rather empty overnight crossing, slept on a sofa in the bar area (well away from where they were serving. I wasn't alone. In fact I was in the minority by sleeping just in my clothes with my head on my bag - pyjamas and blankets or even sleeping bags were common. I did make sure to have an eye mask and earplugs.