While there is no direct indication in the movie of this comment of Becky's, we can assume that, as Mama was talking about her appearance, she also made an excuse of her tomboyish look to make Mama comfortable in her presence.
Also, there can be a lot of things that Becky might be referring to wittingly.
This response suggests Becky is not judgemental.
"Well, I haven’t always been like this.” What does her response
indicate about Becky? What is the message to the viewer about people
This response indicates that Becky isn't judgemental. She excepts
people for who they are. The message that is explored about people in
general is that they can be very judgemental. Society is quick to
judge a persons' apperance; illness or disability before they know the
Or, she used to be judgemental, but not anymore, hence "Well, I haven’t always been like this.
Earlier Becky said to Gilbert that she does not look for external beauty in people.
I'm not into that...that whole external beauty thing, you know. 'Cause it...it doesn't last. You're, you know... Eventually, your face is gonna get wrinkly...you'll get grey hair...your boobs might sag. You know? So what? It's what you do that really matters.
Source (I couldn't find a better script).
Maybe she didn't always look beyond the looks in a person, but now she does.
She also mentioned about her parents being divorced. Maybe she had a rough childhood, but now she doesn't feel that bad. She is living her life to the fullest and helping others to get back on the track. She is even happier to find a soulmate in Gilbert. Maybe she referred to that happy state as in "Well, I haven’t always been happy like this".
These are all things we can speculate that she could have meant by her response.