Yes it can be bad for a toddler to drink too much milk.
A toddler should drink enough milk to keep them healthy and build strong bones because milk contains calcium which is what builds strong bones.
The calcium also helps build strong teeth as well.
But if the toddler drinks too much milk then it can be too filling and then the toddler may not want to eat other foods that contain iron.
If the toddler does not get enough iron then it can lead to anemia which can then eventually sometimes lead to kidney disease and kidney failure.
Iron Deficiency is pretty common in toddlers who drink too much milk.
So just make sure the toddler is drinking just enough milk and have them drink, juice, water, tea etc and make sure the toddler is eating foods that have iron in them so they are not prone to Iron Deficiency.