Probably not. The MX records are listed in the non-editable section.
I suppose you could add your own MX records where you can set the priority and then re-add as a custom MX the DreamHost MX records with a new priority.
[color=#CC0000]However I have not tried this so more than ever I point you at my keep-out-of-trouble clause below![/color]
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!