Honestly, if someone gets butthurt if I say I don’t like certain music, that’s a them problem. People tie their identities very strongly to their music preferences and they sometimes take it as an assault on their character. What you said in this case was a valid observation and I don’t think you should assume any responsibility for their reaction.
But if you’re trying to learn to be more diplomatic, something like a simple, “eh, that’s not really my thing,” gets the point across just fine. It’s an “I” statement which avoids putting people on the defensive. Works for just about anything.