Learn English – What does “get down on someone” mean

I propose two possible interpretations.

Here's a larger portion of the lyrics.

And I'm pretty, pretty cute and I'm pretty smart
And when I break it down, it's a work of art
And if you feel the same, can you participate?
I wanna see you shake, I wanna hear you say
And I'm pretty, pretty cute and I'm pretty smart
And when I break it down, it's a work of art
And if you feel the same, can you participate?
I wanna see you shake, I wanna hear you say
(Direct Lyrics)

One basic interpretation is as follows.

break something down
to discuss the details of something by examining its subparts.
Let's break this problem down into its parts and deal with each one separately.
Breaking down complex problems into their components is almost fun.
Let's break this issue down and discuss it.
(The Free Dictionary)

Notice that she's analyzing herself in the lyrics and she concludes that her traits are a work of art.

Another meaning of break it down is to dance, especially, energetic, loose, freestyle dancing. Here is an example: Ellen and Michelle Obama Break It Down.

So, another interpretation is when she breaks it down (= dances), it's a work of art.

off
adjective
33. slightly abnormal
34. not up to standard; not so good or satisfactory as usual; inferior or subnormal

It's between these two definitions. If I say, "He's been off with me," or, "He's been very off with me," it means that things are not the same between us, that he hasn't been treating me like he normally does, that there's an awkwardness, issue, or something going on with him that's affecting his behavior towards me, like maybe he's upset with me about something I said or did, which I maybe don't even know what that something is because I didn't realize he took offense, got hurt feelings, or had whatever emotional reaction he had that's making things off between us as he maybe hasn't told me, or maybe he's just been in a funk or in a funny mood lately about something completely unrelated to me, but whatever it is up with him, the way he's been with me, the way our relationship has been lately, has been not like it usually is, and not in a good way. It's that strangeness in him and how it's affecting how he's treating me by his treatment of me being substandard that I'm commenting on.

Being that the usage is somewhat slangy or informal, I believe this definition from Urban Dictionary sums it up, as well:

4
off
To be weird, off center, not normal.
"she's a bit off"