It should have a taste more like broccoli but kinda different but also a very different texture, if it's crunchy it's more bulky and crunchy than lettuce, if it's cooked, well, you don't usually cook lettuce. Napa cabbage is kinda thinner so it was probably that

Anyway i wish i could cook for you. One of my coworkers is an older lady who frequently tells me that i'm the only person who can get her to try new food, she didn't like beets but she likes my beets, she didn't like brussel sprouts but likes my brussel sprouts.

Are you vegan? If so have you had them steamed and drizzled in garlic butter (margarine)? What about the texture in particular do you not like? Do you find it bitter?

There's a lot of bitter compounds in brassicas including iirc oxalates in broccoli but blanching can help wash some of the water soluble stuff out