Though @EzekielRaiden I think that is a thing to consider. I still prefer 1d6 then 2d6 at 11th level, but for your version is it too strong to have Blue Dragon Sorcerers or Clerics who got this via a feat to have 1d6+3+3 damage? I don't think it is that bad, but would cap at 1d6+11 if they have a way to add mod.
Personally, I don't think so. Again, lightning is a somewhat weak type for damage, as it doesn't have the higher potency associated with fire, nor a stable of enemies weak to it (as noted, there is literally only ONE official creature in 5e weak to lightning damage, and it's in a single adventure, not even a regular monster book.)
Overall though, 3d10+5 (average 21.5, min 8, max 35) is still going to outshine 1d6+11 (average 14.5, min 12, max 17). It has slightly higher minimum damage, but less than half the maximum damage, and the average damage is about 2/3 that of
fire bolt. Admittedly, due to the absence of thunder-type dragons (which has always confused me, given the excess of fire-based ones),
thunderclap specifically requires an Evoker wizard to maximize, though
acid splash offers a very close alternative since...you REALLY shouldn't be having more than 2 creatures engaged in melee if you're a squishy Sorcerer, and even 2 is highly undesirable.
More or less, we're looking at whether folks who highly specialize in dealing damage, with a spell that is inferior for doing damage in any situation
other than having two or more enemies adjacent to you, are better than a perfectly average character with no specific specialization using a reliable high-damage spell. That seems like a rather biased comparison; of course someone who's specialized in lightning damage (if a Sorcerer) or in generically doing damage with Evocation spells (if an Evoker) should be getting more oomph out of this. They'll also be getting more oomph out of other,
better cantrips, e.g.
lightning lure or
shocking grasp for the Sorcerer or
fire bolt or
ray of frost for the Evoker.
It's more of a "niche" cantrip. You are 100% right that for 90, 95% of casters it's a bad pick. But in my drakkenheim campaign I have a warforged amorsmith artificer who's not afraid of the front line - to him it makes sense.
I mean, I wouldn't put this cantrip on any spell list except Sorcerer and Wizard, so that's not really an issue. A Bard that sinks
magical secrets, or a character that blows a whole MC class level, into picking up just this cantrip is paying top dollar for meager benefit. No one else would have access to it, because it's homebrew. Hell, I might even say "only Sorcerers and Evoker Wizards can learn this spell," to
really prevent any shenanigans with Bladesinger or Magic Initiate or the like.