The Merchant of Venice is a comedy in the sense that what happens in the story is something that could easily happen in real life but in an exaggerated form. Take act 3 scene 1 for example where Shylock explains why he wants Antonio's flesh. People are vengeful animals for the most part and would hurt others for the mere pleasure of revenge. This is the metaphor behind Shylock wanting Antonio's flesh, it is a real emotion just exaggerated a little.
When Antonio is being judged by the duke and Shylock is trying to get his pound of flesh that can also be seen as an exaggerated but realistic event. Also the scene where Nerissa and Portia test there husbands by asking for their wedding rings disguised as men is a thing many women do just not in such a funny way.