Its not that it looks funny. A person should ride what they like..Many say, the problem is the extra drag from MTN. bikes. Can be minimized by putting slicks on your wheels.
But still you are pushing a bike not meant to be used for long distance cycling.
Since I knew from the get go , I wanted to commute, I choose a responsive bike and immediately favored a road bike. IF they hassle you about a MTN bike, thumb your nose at them...But, for road riding, eventually you might want something lighter.
On the other hand..I recall seeing roadies training on MTN bikes..That extra drag makes them stronger for getting on a road bike and racing.