Great question, some would consider it spam. If I had developed a comfortable relationship with the buyer, through questions, answering their concerns and they left me great feedback I would definitely convo, a short but sweet thank you.
If I had to think about it twice about sending them a thank you convo, I would not send, meaning the relationship was just a normal, seller to buyer interaction.
The positive result about great feedback it gives future buyers confidence from purchasing fro your Shop.
Keep focused and keep listing items and tweaking your Shop, as we all do daily and your feedback will flourish. Good Luck.