Honestly, not excessively strong, just annoying and sometimes difficult to coordinate against.
As soon as you see a maokai in the loading screen you should plan for one of two strategies: Avoid the bushes since they'll kill you, or have a tank build MR heavy and body sweep them. Maokai's poke isn't that bad if you build against him appropriately (believe me I'd rather face a mao than a ziggs) and his damage is actually relatively weak unless he builds squishy, in which case he is easy to burst. Alternatively he may have set up bush traps, which are a pain to fight, so your team should never be grouped entering a bush vs him, again preferably sending your MR stacker in as a sweeper to check.
It can definitely be frustrating to deal with him at times, but item wise (since you can't counter pick with champs obviously), I'd recommend rushing SV, banshee's, or maw (depending on your champ), and then having the designated stacker go for adaptive helm and maybe an abyssal, or a non HP MR item too. After about 100 AP, 6 saplings can kill a non-mr-stacker, but since the ratio is % HP you can stack raw MR and be effective at surviving it without sacrificing as much damage. Death's dance is a wonder item vs the saplings too, since it can turn what would kill you in 2 seconds into a 5 second total bleed out, giving you more time to lifesteal, heal, shield, or otherwise be saved by allies (also great in general vs most bursty mage types that are strong in ARAM).
So yes, he's strong, but not brokenly so. If his other abilities actually had decent ratios or he had more overall range then I'd say he was too strong, but for now I think he's manageable.