You can't just counterjungle him and hope that's the end of it. Mumu is supposed to dive in, initiate, and let his team come in to finish the job. I've kept a Mumu under level 6 in a game a while back and everyone else was 10+. As soon as he hit 6 with basically no items, all he did was ult, die, and win. The fact is that Mumu scaling doesn't matter when his kit is all about utility, so keeping him underfarmed doesn't mean much in the long run; you have to beat him with comp and positioning.
I always try to double up on AP when fighting against Mumu, and it helps when both have AoE ults (Cass and Ori are my personal favorites with Swain, Morde, Kat, Vlad, and Ahri following up). Remember that his Ult doesn't stop you from using abilities, so tanky AP champs who can counterinitiate allow you to brush off his ult then win the fight with more damage. High mobilitiy also works to prevent your team from getting hit. If the Mumu was smart in that case, he'd wait until the fight was decently underway before ulting 1-2 key targets and hoping his team focuses them. Your job is to take out their squishies before he gets a chance to do that.