Well, it depends. If you’re drinking matcha for the health benefits, it’s definitely a bad idea. What happens is that the caseins in milk “stick” to the polyphenols in matcha, and make them bio-unavailable (ie they get peed out instead of absorbed by the body). But if you’re healthy and don’t care as much, and just enjoy matcha with milk, go for it! The bonus is that you can, and should, use a lower quality / less-expensive matcha for milky drinks, because the fats and sugars in milk will mask the astringency and bitter taste of culinary matcha and actually make it taste good. It’s a waste to use milk in hyperpremium matcha though; you’ll most be tasting milk and will miss the many lovely nuances of fine matcha.