You've taken the first step by addressing the problem, which is good. The thing to keep in mind when fighting Ike is that his moves have incredible range. You can eat a lot of damage by getting hit by well spaced Forward Tilts/F-air. You should play in a manner that avoids those moves, while still throwing out your own forward tilts/f-air's to rack up damage. Marth is exceptional at midrange, and he should be soundly beating out Ike at mid range due to his faster speed/equally solid range. A good deal of Ike's moves are horrendously slow. Slow in start up and slow on recovery, so he's susceptible to being punished (if you're too far away to punish with a tilt, a dash grab should suffice).
Secondly, Ike's recovery is linear—exploitable. You should be able to heavily damage him with spaced F-air if he's using his recovery specials (remember to avoid Aether range), and even KO him if he's at a high %. I know it might harder in practice to KO him while he's offstage, but that's arguably your method of KO (outside of landing a lucky tipper forward fsmash). Finally, it's worth noting that Ike's mobility is really poor; he has a subpar run speed and his air mobility is poor. Marth on the other hand, has good mobility and is nimble in the air. You should be able to make Ike come to you and punish him for recklessly pressing buttons.
Basically, Ike relies on the opponent making mistakes to win. If you play defensively and safe, you should do significantly better in that particular matchup.