Are intralesional tetracycline injections effective for malar mounds / festoon?

To my knowledge, there isn't any good evidence for this particular treatment. While the risk of an adverse outcome is fairly low, the chance of a significant improvement is also fairly low. Sorry!

Festoons and malar mounds have been a frustration for surgeons for many years. Thanks to the initial work of Dr. Sterling Baker (a person with whom I trained) and my own improvement and refinement on that work over the last decade, I now have a sophisticated, technically-challenging but effective treatment for festoons and malar mounds.
Many people don’t fully understand or recognize the differences between these two conditions. The first difference is in their cause or etiology; festoons are a result of sun-damaged skin on the lower eyelid and on the cheek and are believed to be affected by underlying contrasting muscle forces over the years of a person’s life.
In contrast, lower eyelid bags are due to age-related fat protruding through the skin in the lower eyelid region. New research has shown that fat can grow and atrophy during our lives in asymmetric ways. The growth of fat in the orbit around the eye along with atrophy of fat in the facial regions around the eye, as well as weakness in some of the orbital containing structures, contribute the fullness we see in the lower eyelid with age.
For some individuals, this fat shows up at a very young age and this is believed to a result of genetic changes in the development of the eye area in those individuals.
The second difference is the location on the face where they form. Lower eyelid festoons can be seen primarily on the cheek with some presence in the lower eyelid region.  Lower eyelid bags are located directly below the lower eyelid lashes and are noticed to become more prominent when someone with these bags looks upwards.  Conversely, festoons are minimally affected when a person looks upwards.
The third difference is the feel of these conditions when they are touched. Festoons are squishy and can be made to move side to side. Lower eyelid bags are firmer, deeper in location beneath the skin and can’t be moved easily side to side when touched.
 It is important to understand the differences between these two conditions. The best rejuvenation and most natural results occur when both eyelid bags and festoons are corrected simultaneously.
I'll include some links below.
I hope this was helpful. 

There are many things you can do to treat under eye bags, but a combination approach with nonsurgical treatments would give the most improvement: fillers (Belotero, Juvederm Volbella), lasers (Fraxel/CO2), Renuvion (J-Plasma), microneedling/PRP, Cosmelan peels and stem cells. A surgical option to remove skin and/or fat might be the best solution long-term. I suggest only going to an expert and posting pictures or seeking a formal consultation so we can determine if you are talking about festoons, bags, hollows/circles etc. as these are all treated differently. , Dr. Emer.