I am new to Yoga and some poses are a bit awkward. My instructor says it is more important to focus on breathing at the beginning, and later down the road the poses become easier.
But I’ve read around the net that doing poses with bad posture is harmful!
Should I be worried?
You should never be "worried’ as it serves no purpose whatsoever. Discerning, on the other hand, is quite an appropriate state for one on the yoga path.
It would be most unusual to be a beginning yoga student and NOT find the poses awkward, to say the least. Doing the poses (asanas) over time and doing so without an attention to and refinement of alignment can, in fact, lead to some issues especially with connective tissue.
For this reason students who are stiff or find “awkward” in this pose or that pose are modified so that they can find an expression of the pose that fits them, move toward the plane of the pose, and thus close in on “alignment”.
However it is not every “style” of yoga that modifies poses. In some practices the student must fit the pose. Since you’ve not at all mentioned which of the thousand styles you are practicing it is difficult to say more.
One clarification in closing - dysfunctional posture typically refers to the spine’s position(s) when standing (or sitting). We would do well to bring this concept into each pose. However the current perceptions seem to be that posture is the position of the spine when sitting or standing.
Doing many things with poor posture is risky. Carrying dog food, picking up a child, working at a computer, reading in bed…
So generally speaking I suggest beginners find an alignment-based practice then not worry too much about the things that come (and ultimately go) in the beginning of their practice.