Answer is no you are underclocking it not overclocking meaning you are not pushing you hardware beyond its bounderies. Also what is the main reason that you want to underclock your GPU? For Science maybe?
Underclocking and setting a max temp threshold is a great way to keep the temperature low on your GPUs. If you are in less than ideal situation for airflow or it's a super hot day out or whatever it's a great option for keeping your cards from potentially overheating. Doing this also extends the life of the cards. I did this over the summer, now that winter is here I'll make some adjustments.