Is it unethical to "mislead" partner if s/he is the weakest player at the table?
Playing cards in such a way as to not accurately reflect your hand is perfectly fine - as long as that's actually what you're doing.  There are plenty of hands where you're on defense and you know that partner is pretty much worthless; you're defending 3NT, partner knows your long suit, and you have 14 points anyway and expect partner to be in the lead exactly zero times.  In those cases, play however you wish: you can mislead declarer, or just play at random, any of that is fine. This is called falsecarding, and is a part of bridge.
Where it becomes unethical is when your partner knows to expect it, and can benefit from it.  Again, if your partner can't benefit from it, then it's okay - but your falsecarding shouldn't have an actual meaning (&quot;When I'm the strong hand, defending NT, then I'll play 3rd from length instead of 4th from length&quot;).  It has to be something partner can't anticipate, or definitely can't benefit from.
For the most part, it's more relaxed than psychs; psychic bids are nearly always giving partner information when they can be anticipated, since bidding not only is a process partner may be participating in, but it could also inform the lead or other plays later on.  Just be careful that it doesn't become a habit that can convey unauthorized information to your partner, or else you'll have an implicit partnership understanding that needs to be disclosed.
I would add that the specific case you describe - where you think partner is just incompetent and won't understand the signal, and thus you can falsecard without adverse effect - is probably legal, but gets a lot closer to unethical, and is extremely damaging to the partnership.  Even if I were playing with a paid professional, for example, I'd be extremely upset if they decided to start intentionally faking defensive signals on the theory that I wouldn't notice them, not to mention the fact that I would not be able to improve my own play.