I decided to search for three forms on the Corpus of Contemporary American English (COCA). Here's what I found:
First, I searched for
un-american. This search was not case-sensitive. There were 560 results, and in 560/560, the 'A' in un-American was capitalized. (Well, 559/560, but when I checked the actual source of the odd result out, it turned out to be an error. The 'A' was capitalized in the original.)
Next, I searched for
unamerican. Again, this was not case-sensitive. This search had only 31 results. Of these, 27/31 capitalized the 'A'. Of the remaining four, I was only able to verify that one actually
used the lowercase 'a' in print. (The other sources weren't immediately accessible, but it didn't seem particularly important, so I didn't bother tracking them down.)
Last, I searched for
un american. This had only 7 results, and at least two appeared to be OCR errors. I didn't investigate all 7, again deciding it wasn't important enough to bother with.
So it looks like you're right―you probably should capitalize the 'A'. It also looks like there's a strong preference for un-American over unAmerican, so if you want to stick with convention, that's how you should write it, too.
When you apply the prefix un- to a word, two spellings are usually possible: with and without a hyphen. And over time, spellings tend toward the latter. But this word has been around for quite a while, and the hyphen spelling is still going strong! The OED dates un-American back to 1818, and it's certainly been in regular use at least since the 1930s, when the House Committee on Un-American Activities (HUAC) was established. So it certainly seems like in this particular case the word has resisted losing its hyphen over time. I can make two guesses as to why:
- It's part of the canonical spelling of the aforementioned HUAC. Since this is one of the primary uses of the term, it may reinforce the spelling each time it's named.
- There's pressure to capitalize the 'A' in American, but English orthography has relatively few words with only a non-initial capital letter, and most of those are relatively recent (e.g. iPhone). This is probably why you think it looks "clumsy".
Checking COCA for
un-british (13) versus
un-french (6) versus
un-japanese (12) versus
unjapanese (1), I found a more general pattern, apparently ruling out explanation #1. (Recall that these searches are case-insensitive.) When you apply the prefix un- to a word that's usually capitalized, it seems that it's usually (but not always) spelled with a hyphen. So I suppose explanation #2 is the best I've got: the orthography resists spellings with only a non-initial capital letter.
In any case, since there's a clear convention and you dislike the alternative, you might as well do what most people do: write un-American.