I don't think Koharu's voice is terrible... she just had problems singing right notes. Everybody has problems first. Just give her a chance.
And maybe she's not used to sing with so strong voice which comes from your stomach. I know I sounded horrible when I first tried to sing with a strong voice. One may try too much and end up shouting or just sing offkey. At least I did. It just needs some time and practice (and yeah I know she's been practicing about a month).
...But she's so cuuuuute!
Oh, and there were both songs, title track and c/w, for all you confused people ^^ The one Reina was singing was c/w. Both sounded okay to me ^_^
Episode was nice. Sailor Kame was my favorite <3