The thing with Bieber, Kesha and Gaga is that they are pop music, and just like how pop music intends, they're simple and pretty catchy under the right circumstances.
I don't much enjoy any of the 3 artists you listed in general but "Baby" is probably one of the catchiest songs I've heard and I do like it a lot. Same with Tik Tok, Telephone and what not and for the purpose they serve, I say they're pretty darn good. Personal taste doesn't serve as good objective judgement on what's good and bad, right or wrong, blah blah blah, not to mention the experience of listening to music is very subjective, so......don be hatinnnnnnn
(Friday is definitely trash though)