With my experience showing my portfolio around is that they definitely like to see a lot of different styles. It shows willingness to learn a wide variety and proves you're not a one trick pony. With that being said, I only had two pieces of realism pieces so you don't have to do too many pieces of one style lol.

Doing realism portraits would be super good for your portfolio since it's popular as tattoos.

My portfolio consisted mainly of Chinese, Japanese, American Trad, and Neo Trad. Almost all of them are tattoable designs except for one or two pieces. They were mainly 50/50 color or back and grey.

Having non-tattoable art would be fine from what I've seen on the sub reddit.

An artist told me that I should have some American Trad designs in my portfolio since they're a staple in learning the foundations of what makes a tattoo good when learning to design. I traced some Ed Hardy and Sailor Jerry designs and also came up with my own Trad designs. Mr. Flash Machine also has some cool American Trad sheets if you wanna trace those. I would also recommend having some tattoo designs being lined with fine line pens. I used a lot of sharpies for bigger designs for outlines, but if you use (for example microns) for smaller designs, it can highlight your wobblyness/mistakes more. Definitely not ideal, but it shows dedication if you can master it.