Verb forms in "Feelin' Myself" (will.i.am song)

Solution 1:

I would discourage you from analyzing or learning from a pop song, as grammar is usually bad, and often intentionally so.

For example in your example,

Look up in the mirror, the mirror look at me

This is probably just to keep the flow of the song and not cram too many sylables into one line. In grammatically correct English, it would be "the mirror looks at me."

Similarly,

Cause we be in the club

and

The mirror be like baby

I would say the the use of "be" here, in place of the correct conjugated forms of the verb (are and is respectively) is to make it a bit less of a mouthful, i.e. make it sound better and be easier to say, especially in an (assumedly) fast-paced song.

N.B. As mentioned in the comments, this could be AAVE (African American Vernacular English)