I agree that Willoughby is deliberately trying to impress Marianne and gain her approval by parroting her opinions, agreeing with her opinions, or allowing himself to be won over by her opinions. He seems to be very adept and practiced at 'catching enthusiasm' and in charmingly 'acquiescing' to all decisions in such a way that it appears that the opinions were always his own.

However, I do think there is some genuine shared interest there when he says that “of music and dancing he was passionately fond” . We know that he was up all night dancing at Barton Park months before he ever knew Marianne existed. We know how much he values the company of the people at Barton Park, so it's fair to assume that he does actually like to dance. He would also not be able to fake being good at dancing, or Marianne might notice something amiss there as well, but she does not.

After Willoughby leaves, in Ch. 16 we read that Marianne
"played over every favourite song that she had been used to play to Willoughby, every air in which their voices had been oftenest joined, and sat at the instrument gazing on every line of music that he had written out for her, till her heart was so heavy that no farther sadness could be gained;"

Here, again, we are seeing something that could not readily be feigned. If he didn't, or wouldn't or couldn't actually sing, Marianne would not have all these memories of him singing with her, and it takes some knowledge and skill to be able to write out music as well.

I don't want to defend or make any excuses for Willoughby, but I think at least we can say that his interest in music and dancing was quite genuine.

We know that Marianne is very beautiful and has a captivating personality, but if there were truly nothing that was a somewhat genuine shared interest, I can't imagine that Willoughby would want to spend weeks in her company for hours on end every day just for the pleasure of looking at her.