Dent on one nostril was visible immediately after surgery. How difficult would it be to fix this? Would filler do the trick?

The dent is due to lack of cartilage in that area.  Ideally would would benefit from a cartilage graft there.  Fillers can sometimes be problematic in that area and can result in damage to the skin and tissues of the nose making the situation even worse.

Thank you for your question.
This is not an unusual situation. This could improve with time. If it persists at one year, a cartilage graft may be a better option than a filler. All The Best !

Dear Madam
Thank you for the questions and provided information.
These dent appereance is normal after the rhinoplasty surgery due to anotomical reasons.İn addition with angle of the light coming in to this situation could be more prominent .You should keep get in touch with your surgeon.
I wish the best for you.

Thanks for your question and photo. What did it look like before your surgery? Is this new? Contour irregularities in this area can be from a variety of causes related to the soft tissue, the suturing method used, cartilage contouring and scar tissue formation. Without a physical examination, the cause can't be confirmed. There are some early treatments you may benefit from in this area, but I would not recommend filler. This is an extremely sensitive place to inject filler, and you have the additional factor of recent surgery. Some surgeons inject intradermal instead of subdermal in these areas, but I would not feel comfortable doing that for you in this situation. 
Lastly, I agree with your observation that you are still very early in the healing process and swelling can cause some contour irregularities that resolve over time to an extent where they are no longer noticeable or bothersome to you. Warm regards.