Well, I must confess it's a very interesting and honest question. The first thing I knew about Mika was his song "Relax" and more exactly a part of his song but I already loved it. I love high-peeched voices for men ! And then, some day, he was invited on TV, I discovered as well his physical apperance and also his attractive personnality and I'm still unable to say what I prefeed in him. But what is sure is I was immediately hooked and can't stop thinking about him since that very moment ! I don't know if its because of his looks, voice or personnality. I think the three elements are part of him and cannot be dissociated (not sure this word exists in English !).
What I want to say is that Mika is beautiful but it is not the essential ! It doesn't spoil anything of course but I think I would love his music even though he wasn't. His music always makes me feel so fine ! His lyrics are so intelligently written ! And his voice, especially when he sings "over my shoulder" does thrill me ! It would be the same if he wasn't handsome.
For example, I never though Freddie Mercury was beautiful but I love his voice !... Well, I never went on forums and fan-clubs of his, in fact ! Oh, difficult question, indeed !
When I was younger, I was as hooked on Prince that I am on Mika now and everybody kept telling me Prince was very ugly ! Same thing happened later with Eminem ! I think when you like somebody, he becomes better-looking to you than he actually is. So I would say that if Mika was not so attractive, I would like him as I do now !
And if he happened to have some horrible accident or disease which would totally change his appearance and made him suddenly ugly (which, I hope, will never happen ! Poor MIka !!!!) I would still remain one of his most convinced and faithful fans !