This character on SpongeBob was just a school boy who wanted to wear a girl's bra - not to pretend to be a girl, but to just, well, wear girl's underwear for no apparent reason except that he felt like it, and outside of his clothes, as well.
I believe there is a difference when I have to explain to my child what she is watching, and there isn't an explanation that fits with her age. Of course, I don't let my kid watch just anything on television, so I guess if you don't care what your kid learns from TV, this issue won't bother you a bit.