I don't actually believe this for one moment, as part of the wrenching tragedy of this novel is that the dream of Lennie and George is actually achievable and is not a wild escape from reality with no possibility of ever being achieved. Note the way that other characters are themselves seduced by the dream and its power. However, if you were going to argue this point of view (which you can definitely do), you would need to focus on the way in which the dream of Lennie and George sustains them through the difficult times and loneliness that they face as itinerant ranch workers, and even when it becomes absolutely clear that their dream is lost forever, Lennie still wants to hear the dream. One of the most tragic moments in the entire book is when George recites the dream one more time just before killing Lennie.
Secondly, you can also talk about the scepticism of other characters about their dream, especially what Crooks says to Lennie about their dream and how impossible it is:
I seen hundreds of men come by on the road an’ on the ranches, with their bindles on their back an’ that same damn thing in their heads . . . every damn one of ’em’s got a little piece of land in his head. An’ never a God damn one of ’em ever gets it. Just like heaven. Ever’body wants a little piece of lan’. I read plenty of books out here. Nobody never gets to heaven, and nobody gets no land.
This quote is very important for a number of reasons, however principally it represents a note of reality, reminding Lennie and us as readers of the fact that their dream is just a dream at the moment and of the many barriers that face George and Lennie. It also paints a rather disturbing picture of the world as being a place where others try to destroy your dreams and ruin them, even those who are weak and exploited themselves.
I hope these ideas help you argue your case. At the end of the day, it is definitely legitimate to argue that dreams in this novel are about escaping reality, as the grim and bleak lives of the characters show.