I don't really even know if I should wrote this as an answer, but it's something I've actually thought about before reading your question so I had to take the opportunity to say it since it's likely the only time I'll run across people getting deep about this (admittedly not-so-deep) song.
WWhen I first heard the song, it of course opened with her saying the phrase in question. Having heard nothing of the rest of the song yet, I initally took it musically. It is a common thing for people to say they are "all about that bass", to where the phrase is almost like a sample, except there's no actual recording where someone said it that everybody knows and uses. So, I thought nothing much more about it than just the appreciation of a familiar sample used well. Until she's like "no treble". People dont really say they're not about treble, just that they're all about bass. I initally took it as her just saying that in an extension of being all about bass, while attempting to be witty/cute. To myself, I thought, "no treble? That's stupid and makes no sense if you think bout it, since there's not much bass in the song really anyways"
Once I heard the rest of the song, I realized that she probably meant her butt, but then "no treble" made absolutely no sense. I never even thought about breasts or the upper body, I was just like, "what the heck is she talking about; treble?"
I started to feel like she was just simply guilty of just saying "no treble" because of how shes all about bass and treble is erroneously thought of as being the counter or opposite to bass,( like how capitalism is errobeously considered the opposite, or counter, of socialism), and that pissed me off a bit to where I'm thinking "whats wrong with treble? Im all about bass, but treble has its place." Hahaha.
Upon deeper and more thourough analysis, I have come to the conclusion that she perhaps thought of it originally as a knee-jerk response to saying she's "all about that bass" (while writing the song i mean. FYI, her and some other guy wrote this song together somewhat quickly, starting from something she had previously wrote or some idea she had had. Yes, i actually spent time researching this...) She might have been thinking of bass in terms of meaning booty at first, but once she said "no treble" and liked it, bass as booty leaves the meaning of treble somewhat unclear. So, whether that is how it happened or not, I believe she has decided that bass and treble refer to large and skinny body sizes, respectively. When she says she is "all about that bass; no treble" she is referring to herself and speaking as if to a potential "love interest" and saying something like, "I'm a big girl; you ain't gonna find no skinny bitches here."
The song plays around with "hating" on skinny chicks, calling them bitches then saying she was just kidding, then starting to talk "shit" by starting with "I know you think you're fat" and even though she just did it, you don't think she's joking again and dont expect her reassurances of how it is a given that "every inch of u is perfect"
But yeah. She means bass to mean big, large, etc and treble to mean thin, skinny, etc. But she also thinks of it as referring to booty, specifically a substantial booty. In this regard, she is not thinking of a treblethat refers to the body part which is the counter or opposite of a butt, whatever that would be.