Any blonde ale should do the trick for this recipe. It will go exceptionally well with the breadiness of the tortillas, and the earthy notes of the portobello mushrooms. A pilsner, brown, or wheat beer would also work well with the flavor profile.
There are many reasons to make your own ketchup. The bottled stuff (even the most top-shelf variety) has an indeterminate amount of sugar and “natural flavorings” — what does “natural” taste like, anyway?