What if someone dislikes pickups (and other vehicles with those size/height characteristics, purchased for vanity) that objectively make life more dangerous for kids playing in the street, pedestrians, bicyclists, and others in smaller vehicles?

Is that in the same category as racism?

If it's done without any attempt to comprehend why someone would make the choice, to interrogate your assumption that "vanity" is the only possible reason for it, then yes. It's a prejudicial evaluation rooted in your caricature of someone's nature.

Of course, it's an elective choice, and in that way very different from prejudice founded on appearance. But the incomplete analysis, and the contemptuous reduction of the target's motivations, are in common with racist tropes.

Car accidents (including a car hitting a pedestrian) is certainly a problem but I just don’t see how someone could genuinely dislike pickup drivers for a reason like that. Seems like a conclusion in search of a reason.

The trend towards BIG is a major part of that. Compare, for example, the classic Toyota Hilux to a Ford F-150. The latter is substantially taller off the ground and, given how much of the driver's view is blocked, much less safe in a residential area than the former.