If the Girl really loves you with her heart, the beard or mustache on your face doesn't matter, because true love never had a reason to love, it automatically happens.
But if she is loving you with some reasons, like by seeing your face or styles or richness, etc. then it is never a true love, it is liking. And almost all of you know, love happens only with one, but likes can be happened with multiples.
See some examples which indicates, her love is not true, its only liking.
1. If you try to flirt some girls in front of her and she never reacts hurt, it is symbol of liking not love. True love really hurt when you see your partners with others.
2. You do any kinds of jokes or fun, she will try to join with full hearts. Even if you do rubbish she will enjoy with full heart your rubbish. It is true love, but if she shows negative, it is only liking.
3. She can bear anything for you and she doesn't complain much. True love can handle any kind of problems for their partner and never feels nothing. If she react negative while taking problems for you, it is liking not love.
This were the most common reasons to see true love.
And as many girls like random styles to wear, it may be possible girls may like random styles of boys also. Mustache or beard is no problem for true lovers.