It depends on what you consider to be a good teacher, and what type of ego they have. If they have a big negative ego then they probably won't make a good teacher. If they've got a lot of conceit they may not have so much compassion and I don't usually believe that teachers who don't have much compassion are good teachers.
What is the problem with the ego? We can actually get the ego on our side if we develop the ego with beneficial qualities. We have to use the ego in many ways. It's a bit like, suppose you're going over to Koh Samui and you don't have a boat, so you build a boat, and you get on the boat and you're going across to Koh Samui, and so what do you do? Do you jump off in the middle of the sea, or do you wait until you get to Koh Samui? Usually most people would wait until they got to Koh Samui, it's a bit like that with the ego because, according to the teachings, the ego doesn't dissolve until full enlightenment, the idea of conceit and "I am." This disappears at full enlightenment.
So it's a matter of using the ego, getting all the beneficial qualities of the ego together and using it for benefit. But, trying to control it with wisdom and compassion, especially wisdom. Especially the wisdom of reflecting on the eight worldly conditions, to understand the Dukkha of getting attached to these particular things like praise, fame, pleasure and gain because they're empty of any lasting fulfillment. So to not lose the compassionate intention, and to continually work on one's self rather than to thinking that just because one is a teacher one can forget about this.
The problem with the ego is that if it gets a lot of praise, gain, pleasure or fame it can lose sight of it's original intention and then become content with this particular level. The Buddha uses an analogy in the scriptures of a particular monk who has lost sight of the original intention and settled for a lesser goal. He said this person was like a Dung Beetle, sitting on a pile of dung and saying, "Look at all my dung, look at all my dung, I'm wonderful. I'm the most important dung beetle there is." And it's very easy to look in the world and see people who started out with a compassionate intention, then got attached and content with the eight worldly conditions, and then went astray. It can be really deadly.
We have to try to have enough compassion for ourselves to not get caught in this if one finds oneself in the position of being a teacher.