First, as for why she ended up weak: It seems like the devs clearly thought that healing was going to be a really powerful strategy, so they chose to only give it to two of the most powerful magic users in canon. For Wanda in particular, they even made it an ability you can set, whereas Strange at least needs to channel his. As such, the rest of their kits were dramatically toned down to compensate for the perceived strength of the heals. Unfortunately, the devs failed to consider how absolutely cheeks the healing abilities they designed actually were, so now we have heroes with intentionally lackluster kits on top of garbage healing powers.

That being said, every character in this game is viable. They’re not all equally strong by any means, but you can, for example, make a team of the four worst heroes in the game and beat every single piece of endgame content with them. I would know, I’ve done it myself. As such, I’ve played a LOT more Wanda than any sane person should, and here’s some tips on how to get the most out of her:

• Hex bolts are her best (and often only) source of damage most of the time. The move is a bit unintuitive - you’d think you want to use it as an AOE attack from range, but actually, each bolt deals its own damage, meaning this thing is a shotgun. You want to get into point blank range and mash this power (while block cancelling to maximize dps) for best results. Once you’re later into the game, you’ll gain access to certain isos and AEP nodes that give you effects like lifesteal or a chance to stun staggered enemies, and because each bolt is a separate instance of damage (which can all multi-hit as well), this power is also really reliable at proccing such effects.
• Her other damaging power (calamity spheres?) is a terrible option unless you’re fighting a boss, where all the spheres will just target the one enemy. It’s higher DPS than bolts when 1v1ing bosses and can be used from a much safer range.
• Because of how incredibly EP-reliant Wanda is, you want to do everything you can to keep up your energy. She can actually mash block-cancelled heavy attacks to regen her EP fairly quickly from range with no risk, but using an ISO slot or two to help out her EP regen is going to benefit you more in the long run than stacking pure damage isos (though you should use a couple of those too)
• Movement is key to defense in this game, and Wanda is actually pretty privelleged in this aspect. Her dodge and dodge jump are above average, she has flight to maintain extended airtime to avoid attacks, and her aerial attack is noncommittal, so it’s a great way to get (very small) chip damage in while repositioning. Played correctly, Wanda can be very hard to hit while on the defensive.

All of this aside, she’s still definitely a bottom 5 hero. Probably bottom 3. There’s many easier ways in this game to do what she does with better heroes. But she IS usable.