Assalaamu alaykum. If I get surprised at a person's wit and say, "If only I could see what Allaah made him with," am Ia sinful?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

Allah, The Exalted, created people with His Might and Knowledge, and He created Aadam (Adam, the first man) from dust and then created his descendants out of semen - despised water. If your statement is a remark concerning the first fact (that Allah created Man with His Might and Knowledge) and you hope that you could see the Knowledge and Might of Allah, then this statement is closer to prohibited speech than to idle talk and you are required to seek the forgiveness of Allah from it.

However, if you meant that you wish to see the dust from which man was created, then that is considered idle and vain talk, which the Muslim is commanded to avoid. The Muslim is required to guard his tongue and refrain from speaking of what would not benefit him even if it were lawful. He should train himself to speak only of what is good and beneficial and refrain from speaking of evils and engaging in idle talk that brings no benefit. The Prophet, sallallahu ‘alayhi wa sallam, said, “He who believes in Allah and the Last Day, let him say what is good or remain silent.” [Al-Bukhari and Muslim] Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, “The one with the greatest number of sins is the one who speaks the most of what does not concern him.” [Al-‘Uqayli - As-Suyooti graded it hasan (good) as Mawqoof on the authority of Salmaan (i.e. related from a Companion, not the Prophet, sallallahu ‘alayhi wa sallam)

Allah knows best.