3 idiots is a bollywood masala film and let me tell you masala movie does not mean mindless crap movie but a movie which is a mixture of several genres. It is comedy, drama, bit musical etc just like bajrangi bhaijaan.
Now coming to why it was not worthy of oscar nomination :
It talked about our education system, its merits and demerits, but never went deep into the subject. It merely scraped the surface by talking about student’s commiting suicide, rote learning, not following one’s own dream etc. These issues are so sensitive and important that you can make whole movie on a single topic to understand and present them better in front of our society. Taare zameen par showed the whole mentality of a dyslexic child, remember the scene where the boy imagine what is 3*9=? But in 3 idiots, the character of Joy committed suicide and we got some statistics from Aamir khan like which is the best college, no instrument to measure pressure of human brain etc. It never went into the minds of Joy, what exactly went through his mind while taking such a drastic step?? Therefore TZP is an intellectual movie where as 3 idiots is just a fun movie with a bit of message.