Is looking down upon people a sin?

This question can be answered using God's law of love towards Him, others and self. It's spiritually unkind and unloving to say bad things about people. A caveat, though. Sometimes what one person sees as "bad" may indeed be truth, so your terms need to be further defined.

Sins can have different impacts from a physical point of view and experience. For instance, lies cause no physical harm, but murder ends a life. All repented past, present and future sins are forgiven by God with the exception of blasphemy against God the Holy Spirit.

Yes referring to another as worthless, or a fool, is sin. The conclusion of God's Word is that there is one lawgiver who is able to save and to destroy; who are we to judge another? The Lord has concluded that if we suppose our self to be righteous, and despise others, He says this: Everyone that exalts themselves shall be made low, but they that humble themselves are the ones that shall be exalted [in the eyes of God]. And it does not matter what the world does, for the world would rather exalt the proud, and make fun of the humble.

If that were true all Christians would sin daily, particularly the ones on this site.

yes all sins from murder to lies to too many fruit pies are all the same

No it is not. All humans do it

Yao Ming is going to Hell