The main point of the monkey in “Nope” was to demonstrate the consequences associated with making bad choices and facing peer pressure. The monkey was given a choice—he could either take a banana or have a grape.
If he chose the banana, which was offered up by a peer, his situation would become worse as he was left alone and confused in the end. However, if he chose the grape, he would be rewarded with happiness. This is meant to be symbolic of making the right decision when faced with difficult choices and peer pressure.