By perfectly fine I mean: not at all morally blameworthy.
By aiming I mean: being ready to calibrate ourselves up or down to hit the target. I would contrast aiming with settling, which does not necessarily involve calibrating down if one is above target. (For example, if you're aiming for a B, then you should work harder if you get a C on the first exam and ease up if you get an A on the first exam. If you're willing to settle for a B, then you won't necessarily ease up if you happen fortunately to be headed toward an A.)
I believe that most people aim to be morally mediocre, even if they don't explicitly conceptualize themselves that way. Most people look at their peers' moral behavior, then calibrate toward so-so, wanting neither to be among the morally best (with the self-sacrifice that seems to involve) nor among the morally worst. But maybe "mediocre" is too loaded a word, with its negative connotations? Maybe it's perfectly fine, not at all blameworthy, to aim for the moral middle?
Here's one reason you might think so:
The Fairness Argument.
Let's assume (of course it's disputable) that being among the morally best, relative to your peers, normally involves substantial self-sacrifice. It's morally better to donate large amounts to worthy charities than to donate small amounts. It's morally better to be generous rather than stingy with one's time in helping colleagues, neighbors, and distant relatives who might not be your favorite people. It's morally better to meet your deadlines than to inconvenience others by running late. It's morally better to have a small carbon footprint than a medium-size or large one. It's morally better not to lie, cheat, and fudge in all the small (and sometimes large) ways that people tend to do.
To be near the moral maximum in every respect would be practically impossible near-sainthood; but we non-saints could still presumably be somewhat better in many of these ways. We just choose not to be better, because we'd rather not make the sacrifices involved. (See The Happy Coincidence Defense and The-Most-I-Can-Do Sweet Spot for my discussion of a couple of ways of insisting that you couldn't be morally better than you in fact are.)
Since (by stipulation) most of your peers aren't making the sacrifices necessary for peer-relative moral excellence, it's unfair for you to be blamed for also declining to do so. If the average person in your financial condition gives 3% of their income to charity, then it would be unfair to blame you for not giving more. If your colleagues down the hall cheat, shirk, fib, and flake X amount of the time, it's only fair that you get to do the same. Fairness requires that we demand no more than average moral sacrifice from the average person. Thus, there's nothing wrong with aiming to be only a middling member of the moral community -- approximately as selfish, dishonest, and unreliable as everyone else.
Two Replies to the Fairness Argument.
(1.) Absolute standards. Some actions are morally bad, even if the majority of your peers are doing them. As an extreme example, consider a Nazi death camp guard in 1941, who is somewhat kinder to the inmates and less enthusiastic about killing than the average death camp guard, but who still participates in and benefits from the system. "Hey, at least I'm better than average!" is a poor excuse. More moderately, most people (I believe) regularly exhibit small to moderate degrees of sexism, racism, ableism, and preferential treatment of the conventionally beautiful. Even though most people do this, one remains criticizable for it -- that you're typical or average in your degree of bias is at most a mitigator of blame, not a full excuser from blame. So although some putative norms might become morally optional (or "supererogatory") if most of your peers fail to comply, others don't show that structure. With respect to some norms, aiming for mediocrity is not perfectly fine.
(2.) The seeming-absurdity of trade offs between norm types. Most of us see ourselves as having areas of moral strength and weakness. Maybe you're a warm-hearted fellow, but flakier than average about responding to important emails. Maybe you know you tend to be rude and grumpy to strangers, but you're an unusually active volunteer for good causes in your community. My psychological conjecture is that, in implicitly guiding our own behavior, we tend to treat these tradeoffs as exculpatory or licensing: You forgive yourself for the one in light of the other. You let your excellence in one area justify lowering your aims in another, so that averaging the two, you come out somewhere in the middle. (In these examples, I'm assuming that you didn't spend so much time and energy on the one that the other becomes unfeasible. It's not that you spent hours helping your colleague so that you simply couldn't get to your email.)
Although this is tempting reasoning when you're motivated to see yourself (or someone else) positively, a more neutral judge might tend to find it strange: "It's fine that I insulted that cashier, because this afternoon I'm volunteering for river clean-up." "I'm not criticizable for neglecting Cameron's urgent email because this morning I greeted Monica and Britney kindly, filling the office with good vibes." Although non-consciously or semi-consciously we tend to cut ourselves slack in one area when we think about our excellence in others, when the specifics of such tradeoffs are brought to light, they often don't stand scrutiny.
It's not perfectly fine to aim merely for the moral middle. Your peers tend to be somewhat morally criticizable; and if you aim to be the same, you too are somewhat morally criticizable for doing so. The Fairness Argument doesn't work as a general rule (though it may work in some cases). If you're not aiming for moral excellence, you are somewhat morally blameworthy for your low moral aspirations.