Tattoos in general. This style of tattoo is associated with Yakuza here in Japan and most onsens will not allow you to bathe if you have any tattoo period. Y buddy and I drove down to Izu and he was turned away from several of the nxicer onsens because he had a rather large Canadian flag tattooed on his shoulder blade and a flaming heart on his bicep.
The original poster may say "oh no, not my precious hot spring!" but let's not forget that this area is for JAPAN discussion and here the hot springs are a pretty big deal. It's not like back home where we can just get together and jump into a hot tub. Plus, the onsen is a great place to socialize.
So again, no. I wouldn't get one because I enjoy soaking my chunk body in the clean water of a hot spring. I always sleep like a baby after it. Plus i'm not a fan of pain so having a needle jammed into my skin a few thousand times simply isn't all that appealing.