Viola here is talking about Olivia's love for her (Viola). Olivia might as well love a dream because she's fallen in love with Viola while she (Viola) is disguised as a man. (Viola visits Olivia disguised as Cesario to declare Orsino's love for Olivia.) So, to Viola on a conscious level, these words simply mean that Olivia's love is hopeless. However, they also work on two other levels. Since Viola is falling for Orsino and can't share that emotion with him, her love too is like a dream. What's more, since Viola has a brother who appears later the play (and who Olivia can love), Olivia's impossible love really can work out, just like good dreams do.