What's the best treatment for dents on the side of my eyes after lower eyelid surgery with canthopexy? (photos)

The dents are likely due to scarring around the sutures used in canthopexy. Elevating the areas of adherence with a needle (subcsion) combined with filler or fat would have a very good chance of softening the indentations, but may not completely correct them.

I cannot say for sure, but it appears that there is scarring that is causing tethering.  This leads to a divot where it is tethered down.  If this is the case, then fillers alone may not improve the appearance.  You may need subscision (freeing of the tethering) along with fillers.  You should see someone in person who can examine the areas of concern, palpate any scarring and give your eyelids a thorough exam.  The good news is that this should be repairable.