It spoils the joke. Do you think that it is a coincidence that ‘bathroom’ is the last word that the artist uses in the caption? The joke works (as much as it does) by subverting expectations. The artist sets up our expectations with the image, and puts the key word to the whole subversion as late as possible in the caption (in this case the last word). The comic is ‘spoiled’ when this key word appears in the title, before the reader has even seen the image or caption. Simple exposure to the idea of a bathroom being important to the context of the situation of the comic is enough to lessen the impact of the subversion of expectation, or even make the intended subversion seem trite or obvious.