Yes, she is supposed to be the weakest champ (I'm not entirely sure that she is, but she's at least in the bottom 5). This is a part of her fundamental design. A stealth ganker changes the dynamics of a missing opponent dramatically, because any amount of commitment whatsoever can be fatal; there's no chance to see them coming and back off. Here's the way the game normally works (if no one is fed):
1v2: I am outclassed, I should back off.
1v1 or 2v2: even numbers, if I get an advantage I should exploit it.
2v1: We have a clear advantage and should press it.
Here's how it works with an effective evelynn on the enemy team:
1v2: I am outclassed, I should back off.
1v1 or 2v2: We may be at a disadvantage, and I should let small opportunities go.
2v1: At the very least it's even, if I get an advantage I should exploit it.
With this kind of passive threat to all lanes at once, an MIA stealther
should properly be weaker than other champions to balance it out. Additionally it makes them more strategic, because you can't just ambush softies by yourself; Eve
needs a teammate to lock down a kill, meaning she can't
completely win out of nowhere unfairly. If she had comparable strength to other champions, she could walk all over champions whose main defense is range, like Kog'Maw, with basically no strategy or skill required. This "soloing from stealth is unfair" is the whole reason she was changed from a stun to a slow. You can solo people with a stun if you're strong enough, but a move speed slow is not going to help you kill people from full health at all; they can just kick your ass. A slow, on the other hand, locks down a kill very easily with any teamwork.
The effectiveness of an Eve player is directly determined by how nervous and unsure you can make the enemy team. If the enemy is either wasting a
lot of gold of stealth detection, or passing up perfectly good opportunities because of the fear that an Evelynn may be nearby, you are winning as Evelynn. A great deal of this involves headgames with your opponents. One of my favorite tricks is to enter a side lane brush and then stealth - if I think it's warded I stealth straight-up, otherwise I "accidentally" come out just before I become completely invisible. Then I hit my ult and shurelya's to sprint back to mid with my boots of mobility to kill the mid who was just told that eve was in the side lane. The point is that once you have proven to the enemy that you are unpredictable - once you make them
calculate what they can't see, and fear what they don't know, it will drain on them the entire match even if you end up with relatively few kills and assists. The primary tool of all stealthers is fear, and perhaps none moreso than evelynn.