There are 19 creatures in the Monster Manual that are resistant to necrotic damage. By comparison, only 4 are resistant to radiant damage. This makes playing a Warlock or necromancer Wizard, or using the cleric spell Spirit Guardians as an evil character, unfair compared to other classes that can deal radiant damage.
Is there a way to bypass resistance/immunity to necrotic damage?
For some damage types, there is a way to overcome resistances available to most characters. Specifically, the Elemental Adept feat does this for exactly one of acid, cold, fire, lightning, or thunder damage. It's conceivable that there someday be an equivalent for necrotic damage, but it does not exist at the moment.
Jacob's answer shows one method, which is available if your DM has made villainous class options available, and you are okay with choosing a particular subclass. But you may want to consider whether this is the problem that you think it is.
Most campaigns are more likely to run against mobs of undead than mobs of celestials. That's a result of the fact that most campaigns follow good characters, who are unlikely to fight angels but quite likely (for example) to fight a dracolich.
It makes sense that many of these creatures would be resistant to necrotic damage, just as using fire damage would be ineffective against a fire elemental.
The fact that these monsters appear in great numbers in the Monster Manual is a reflection of this tendency. It's not necessarily a reflection of what you'll see in practice. If your campaign doesn't focus on fighting undead, you simply won't run into this problem. If your campaign does focus on fighting undead, it's not as a result of a bias in the Monster Manual, but a result of the campaign setting.
Either way, the solution you should consider is the same. Diversify your damage types. Even an evil mage will understand that sometimes they'll fight a rogue specter or wraith from time to time, and should prepare accordingly. In fact, a spellcaster who is *ahem* intimately familiar with undead will be more aware of this fact than your average mage.
There is a subclass included in the DMG for clerics that allows you to bypass necrotic resistance with your spells and channel divinity. See the Death Domain Cleric, Inescapable Destruction lvl 6, p. 97:
Starting at 6th level, the cleric’s ability to channel negative energy becomes more potent. Necrotic damage dealt by the character’s cleric spells and Channel Divinity options ignores resistance to necrotic damage.
The Elemental Adept feat (Player's Handbook, p. 166) allows a character to ignore resistance to a chosen elemental damage type (acid, cold, fire, lightning, or thunder). Unfortunately, necrotic damage is not one of the damage type options.
You could ask your DM to allow a homebrew version based on the feat:
Celestial Adept
Prerequisite: The ability to cast at least one spell
When you gain this feat, choose one of the following damage types: necrotic or radiant. Spells you cast ignore resistance to damage of the chosen type. In addition, when you roll damage for a spell you cast that deals damage of that type, you can treat any 1 on a damage die as a 2.
You can select this feat multiple times. Each time you do so, you must choose a different damage type.
If your DM allow this option to you, there is a Dark Gift option called Deadly Touch that has a feature called Inescapable Death that allows you to ignore resistance to necrotic damage (on top of dealing additional necrotic damage if you use unarmed attacks and/or grapples or being grappled). It’s a fairly niche option though. Dark Gifts are available in Van Ritchen Guide to Ravenloft.
Touch of Death Your touch is pain, harming whoever you come in contact with. The deathly power within you is beyond your control, afflicting any who touch your bare skin. By the same token, you can deliver death to your enemies with your touch.
Further it adds the following features.
Death Touch. You can focus your deadly touch against your foes. As an action, make one unarmed strike. On a hit, the target takes an additional 1d10 necrotic damage. This damage increases by 1d10 when you reach 5th level (2d10), 11th level (3d10), and 17th level (4d10).
Inescapable Death. When you hit a target with an attack roll and deal necrotic damage, you ignore the target's resistance to that damage.
Withering Contact. When you start your turn grappling a creature or grappled by it, the creature takes 1d10 necrotic damage.
