It depends on your perspective. From one perspective, it may be seen as weird to ride a mountain bike on the road because mountain bikes are built for off-road use and typically have wider, knobbier tires that are not optimized for riding on pavement. From another perspective, it may not be seen as weird to ride a mountain bike on the road because some people do this intentionally in order to get better performance out of their bike. Riding a mountain bike on the road can provide a more challenging workout than riding on pavement and can also help you to better prepare for off-road cycling.