Because it's a children's cartoon, she probably isn't lesbian because angry parents would be attacking Hasbro if they had a gay character.
There are characters in other media that some parents assume are gay and they throw a fit. Like SpongeBob, Barny and many other characters.
If Hasbro took a step forward and made a character gay, it would be awesome for social reasons but possibly bad on business. So any My Little Pony character being gay is not likely.
On a side note, if they make a character gay or bisexual, I'd prefer it to be a more feminine character, like Fluttershy or even pinkie pie, to break the masculine lesbian stereotype and to break the stereotype of masculine women are all lesbians.
A Note on the Kissing Scene. If they did kiss, which I don't believe they did, I don't think it means she is a lesbian. I know many women who are okay with kissing their good friends, even if they are straight.