I get what you mean, I think — you’re talking about traffic fingerprinting. But you can use the same anti-traffic-analysis techiques used elsewhere in systems like Tox. For example, the darknet market itself could use some of its revenue to pay for “noise transactions” (wash transfers through the mixer, then intentionally “black-laundered” in the market) to keep the number of darknet-market-spent outputs constant per mixer step, by asking for advance notice from buyers for when transfers targeted at their sellers will happen, and then running N fewer “noise transactions” during the appropriate mixer steps.
Though also, you’re assuming a constant “your account” in the above. If you mix 100% of your holdings every time you transact, setting it so that a set amount goes to a darknet market, and the rest goes back to a newly-created public-key-hash that you just generated the keypair for — and then when you want to use money from that address, you fully consume it to mix it again — then nobody ever gets the opportunity to fingerprint “your” traffic. There’s no stable “you.”
(I have a theory that this is the goal Satoshi was aiming at with Bitcoin UXTOs, but never finished that element of the design, and launched it half-baked.)
This also means that the mixer gets to eat a percentage fee off of your complete holdings every transaction, so it kind of sucks, but what can you do.