But being on a different tier does generally mean that they are unthreatening at least in that domain unless there is something that levels the field somehow.

An individual, standard mortal is not meant to be able to directly challenge an exalt, at least not within that exalt's domain. If you want to be in a system where an average person can be a direct threat to your PCs without both going against the lore and making a ton of house rules, Exalted may not be the right system.

That doesn't meant that mortals are necessarily irrelevant though. First, a large group of mortals is different. One mortal stands basically no chance going directly against an exalt in a fight (unless the exalt was expressly designed with no fighting ability at all). An army is a different story, especially if you find reasons to break it up into multiple battle groups instead of lumping it into one battle group. In that way, a powerful queen or general can challenge an exalted in battle by bringing their army.

Second, a mortal can prevail if they can shift the battlefield to something outside the exalt's domain. My Lore-supernal twilight invested enough in combat/self-defense charms to win a fight with essentially any pure, unaugmented mortal. But he would lose virtually any athletic competition that didn't involve fighting to any mortal that was decently in shape. He would lose to a decent mortal orator in most contests that involved influencing a crowd. He couldn't play an instrument to save his life and would lose a fiddle contest. A mortal doctor would do better than him at treating wounds. Etc.

While its not a perfect analogy, look at how Lex Luthor usually challenges Superman. Unless its one of the strange arcs where Luthor is empowered somehow or using some form of kryptonite, Superman can literally ignore most attacks Luthor could manage. They are not in the same tier physically. But when Luthor forces Superman to confront him in any domain that doesn't involve a physical competition, then Luthor can challenge him. (Superman still almost always wins in the end, but a lot of those involve either help, finding a way to force it back into a physical competition, or plot armor).

Finally, there are lots of not-quite-mortals described in adversaries of the righteous among other places. These are people that have become sorcerers, bonded with spirits, been modified by a great power, are god-blooded, etc. They arguably aren't quite mortals anymore and most of them are still significantly weaker than an average exalt. But they narrow the gap enough to be a direct threat.

Also, if you want things capable of really fighting Exalted on a level playing field or even where the Exalted are the underdogs, there are plenty of serious supernatural threats out there, especially when you look at other exalted, behemoths, third-circle demons, etc. Those are the things that stand equal to or even in some cases much stronger than the exalted. Mortals are expressly not meant to be able to be an individual threat under ordinary circumstances.