IMO it's pretty much even (i would even say 55:45 Ike but I'm in the minority there
), but I agree with some of the points of what 55:45 Mario people say. 45:55 either way.
Watch out for ike's jabs. Ike knows you're mario, so he knows you can both up B (sounds risky to me!) or nair Ike if you DI correctly, so Ike would usually attempt to throw or shield throw you after a while. Watch out for his backthrow to dash attack, because it can get you offstage very quickly.
Ike kills way earlier than you. Don't get caught in the air and in danger. You don't really have too many moves you can outright get away from ike with because of his nair and even some of his other aerials, so you may be air dodging more than you want, something Ike could capitalize on. Platforms are not your friend vs Ike. Ike fights for his platforms.
One could say that You could space fireballs and get close, but you can't do both at once. if You get too close while shooting out a fireball, Ike could still hit you with fair (yet get hit with the fireball himself) which ultimately hurts you more.
That's why fireballs offstage are not always reliable when recovering from offstage. One hit of fair and you're either worse off or in the exact same position. That is why you can't do much except dodge offstage in order to try to completely avoid him, something that can be predicted, so don't get caught in that position.
Ike's jabs are really annoying for mario. It can cancel out fireballs(not to mention powershield), yet you still have very little time to get inside and lay the damage on. I'm unsure if you guys still have things you can do to people even on powershield, though. You must force Ike to approach in order for you to have an advantage.
Mario has the tools to gimp Ike, but Ike has a way to avoid only one at a time. You can't gimp Ike's aether with cape if it's reversed or he isn't holding forward when you cape him, but Ike also has to worry about FLUDD, which is a lot trickier for Ike to avoid. Bo is right about keeping FLUDD full at all times.
Against an Ike who spaces well, it is actually pretty difficult for Ike to get juggled in the air. Ike's nair is a big pain to get around because the hitbox goes all around him, and well spaced, can't be punished. Getting a kill move as early as possible is crucial because it'll be hard to hit a good Ike that just doesn't fall trap to one of your fsmashes.
CP Ike FD, or stages with little platforms. Platforms <3 Ike and vice versa. That is Ike's worst stage against you. If they ban FD, go to FD2 (smashville). Tricky stages or stages with lots of obstacles and other things you don't want to bring Ike to.
Ike will try to CP a variety of levels against you. Brinstar, delfino, PS1, any gay stage, haha. Watch out for that.