No, not really. Most champions' ults are powerful, but not having them doesn't scream "Please kill me now! I literally can't stop you at all!" for most. Some HAVE no ult, and several have extremely short cooldowns and/or conditional resets that allow them to be available quite frequently.
Ekko is very, very vulnerable without his ult, because his kit is otherwise set up to create small windows of opportunity for moderate burst damage and then slow you just long enough to get away IF he pulls it off. And if he is stunned at any time during this, he's basically toast, because his best escape (his passive) is on a timer and he needs to get that third hit in. Otherwise, he spends all his time running from/around you, throwing Q to try to hold you off, and maybe occasionally going in for that third hit if you get too close, because he needs the speed boost to keep running. If he lands the stun without his ult, his best bet is still probably to run, unless you're already low. Why does he need to keep running? Cooldowns. He has no sustained damage, and he's really squishy. He can't handle a prolonged fight, and without his ult, he can't punish you for forcing one.
Seriously. Ekko is fun to play as, with, or against, but people are making way too much of a fuss. He's not a big deal. He's certainly not OP. Lich Bane, on the other hand...