If you search the word tabard into google, you'll see that it flows down around the knee area. Tabards are generally longer, not the size of a shirt stuffed in a belt. So yes, the bottom needs to be able to flap around. What I recommend is giving the outlining around the tabard a bit of weight. How you do it is up to you, but it'll add enough weight to not have it flow around so awkwardly.
Kirito (Aincrad, not ALO)
Ino Yamanaka 75% (Need wig cut and styled)
Pink Power Ranger (possibility)