Sometimes yes, sometimes no.
In fairness, if they could open folks like Garrus, and Tali to romance, and introduce others, Joker could have been an option as well. (Hopefully, that would not teach EDI the definition of jealousy.) Joker could have given Kaidan a little competition, and certainly been an improvement over Jacob. Plus, Joker's condition would have been an interesting consideration for their relationship.
On the other hand, sometimes I think it's for the best Joker is not an option simply because - in my opinion - he and Shepard are already closer than they could ever be romantically. They hunted Saren together, he managed an unthinkable maneuver to get Shepard onto Ilos to save the Citadel; Shepard died saving Joker when Normandy was destroyed (Oops! Spoiler! :)), they braved the galactic-freaking-core together; Joker stole the Normandy again to rescue Shepard when the invasion started, and Joker delivered the commander in style to save the whole-stinking-galaxy. He has always performed the impossible for his Commander who literally died to save the best pilot in the galaxy. To link them romantically, could complicate and diminish the already strong bond between them.