The mount does not need mage armor. Blink does make you slowly fall through the floor so you should have no trouble staying on the bull. I agree that logically staying on that bull should be an issue, but real life logic falls apart in a lot of places in the game. The spell only does what it says it does. You will have no problem riding a mount.
You don't fall, but you also aren't effected by a mount if it moves while you are ethereal. The spell doesn't specifically address being mounted, but it does mention moving through creatures. I would certainly have it have some negative effect on being mounted, probably either a 50% chance of not going with your mount or reducing the speed of the mount by 50% (you can't move as much because you are only there half of the time.) There is room for GM interpretation of course, but blink on a mount should probably have some effect. I have never encountered someone wanting to do this though, and have never had to specifically consider it before.
Mage Armor on the mount won't help though. Mage Armor indeed does what it says it does, and it that is gives the mount a +4 Armor Bonus to AC that effects incorporeal attacks (even touch attacks.) It doesn't though give the recipient ghost touch. A person with mage armor on them can't grapple or make an unarmed strike against an incorporeal creature and have it be effective. The same principle would hold true for the mount.
I think blink is the spell that gets being incorporeal and ethereal mixed up because it says they are the same, but they are not. It has been FAQ'd, but never fixed.<---useless info
In either case you are going to have problems with staying on the saddle.
It does call out moving through creatures, which I somehow forget about. Mage armor only gives the AC bonus as Dave mentioned so that won't work.