Jirachi is great, make it tier 1 before suspecting if we ever would :/
Anyways, Jirachi also has lots of notable weaknesses that would make me lean no ban. As much as I love it, I find myself deviating away from it because of a few things:
Fire types: they're incredibly dominant in the current meta (name a team without a fire type?), and Jirachi hates fires. That isn't to say "hey duh it's a steel it'll struggle with fires" but other steels, such as heatran or aegislash have matchups that are a little better. Same goes for stuff like Bisharp, to an extent.
Spread: Jirachi hates spread and is weak to many strong forms of it, excluding hyper voice/rock slide. If rock slide was banned Jirachi would be so good, hello jirachi volc. Alas, such isn't the case.
Lack of recovery: it's hard to rotate Jirachi like one can switchspam with Amoonguss because it lacks reliable forms of recovery, hurting it severely.
Playstyle restriction: it's not really a good tr setter (thought it is pretty neato, heatran and aegis -> free subs and SE stabs, plus stalling out turns with iron head is ???), and only really fits on Hyper Offense. When I say it only fits on Hyper Offense, I don't mean you can only build it HO but the way Jirachi works is that if you're out with another passive mon at the same time, things aren't going to be fun. Jirachi enjoys being the 1xbulky mon on the team while everything else is killing things, and this "archtype" or I suppose "teamstyle" heavily suffers from things such as Talon, Kang, just about every "fast things check" for lack of a better word. Tailwind and Thundurus will almost always be giving such teams a hard time, and Jirachi doesn't exactly help this. For example, using both Jirachi and Suicune on the same team is really weird. Using both Jirachi and bulky Thundurus on the same team is also pretty weird. In general, running more support mons or speed-control focused mons along with a jirachi ends up with a passive team that's hard to utilize effectively. As such, at least in my own experience, can't really speak much for others besides what I've seen, Jirachi constricts its teams into struggling against landot and talon, and this two-mon combination, unless you're using jirachi+lando, which is cool but still suffers under the issue of being passive, is a huge teambuilding constraint for Diancie+Jirachi teams, or simply Diancie teams in general.
There's some thoughts on Jirachi do with it what you will n~n