No, they shouldn’t be lowered. I’m against anything that lowers the difficulty of creatures, since they are a threat only when you are on foot, without any equipment and have a new character.
Mi-go’s spawn near scientist loot, they are typically and almost always on an open field, so if you run into them with a poorly prepared character, you have tactically fracked up, because on an open field scenario it’s difficult to find cover or escape. But due to their typical placement and spawn, they are easy to spot and it’s a matter of you properly scouting ahead and being aware of potential dangers having the most impact for effective means of dealing with them. They are a threat if you travel recklessly and without caution.