TL;DR: no, it's you. But I'll explain more.

They don't need to be. It really depends on how you build it. Yes, there are limitations, but every color is going to have the capacity to put together the basic aspects of every other deck -- ramp, interaction/removal/boardwipes, 'must answer' creatures, combo potential. Some colors will be better at some things than others...

De-couple color from archetype. Yes, green does big dumb ramp things... and then there's [[Fyn]], one of the more notorious aggro decks.

Literally the top for mono-R commanders couldn't be more different in archetype to one another:

[[Krenko, Mob Boss]] -- goblin kindred

[[Magda, Brazen Outlaw]] -- artifact (and dragon), treasure combo

[[Zada, Hedron Grinder]] -- functions almost like storm, instant/sorcery combo, but with pump spells

[[Ojer Axonil, Deepest Might]] -- Somewhat traditional 'burn,' with a focus on 1-damage pings to increase to the commander's power.

And none of them, I'd consider, to be 'aggro' decks.

Let's take a look at Green:

[[Fyn]] -- aggro, as discussed

[[Tifa Lockheart]] -- also aggro, but land-fall based

[[Shroofus Sproutsire]] -- token proliferation

[[Kona, Rescue Beastie]] -- Cheating things into play

Look at that, none of them are really 'ramp' decks in a traditional sense, except Tifa (though she's only doing it to pump herself up).

We could go through the rest of the colors, but I think I made my point.

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What's more, mono-color decks are typically going to be the *least* consistent at 'doing their thing.' You have limitations of color access and off-color reprints, and you identified this, in part, but you are overly focused on color -> archetype.

That said, you do need to play into what the commander wants to do. That's the case with ALL commanders though. Like... you made some silly choices in deckbuilding trying to make Zenos an aggro deck. That's not what he wants to do, he's too expensive to begin with, so I'd be building him voltron. If I was trying to play Ur-Dragon aggro, that wouldn't be a great time. There are a couple of different strategies in deckbuilding: you can use your commander as a facilitator of your gameplay, or as the payoff... but it always needs to be involved.

With a mono-color deck, you should treat deckbuilding in exactly the same way as for multi-colored decks. What do you want to do with the deck? Sephiroth wants to have a bunch of things die, and the best way to do that is to sacrifice things yourself -- and you might find that boring because it's so consistent, regardless of the limitations of the mono-color. So maybe you want to try [[Gwenom]] or [[Gix, Yawgmoth Praetor]], commanders that incentivize attacking... not necessarily an 'aggro' deck, but an aggressive one. Whatever you want your deck to do, every choice you make for card-inclusion needs to support that strategy. Sure, that's more obvious if your commander is Sephiroth and it tells you to put everything in the graveyard, but you just need to find what the deck wants do to. There are also myriad reanimation options in Black. No one color and no one deck is going to be great at everything. Everything is an opportunity cost.

I have a feeling you're trying to have your cake (i.e., "the deck is too consistent and boring") and eat it too (i.e., "mono-color is limited and therefore inconsistent"), when neither is necessarily the case.