Yom Yerushalayim and all Zionist holidays are de facto, if not de jure, neutral if not sad days as any Zionist victory is built on their massive failure and tremendous damage they inflicted on us before that (and afterwards).
It would be (somewhat) like making a Yom Tov to commemorate the Warsaw Ghetto uprising. Was this a bright moment in a terribly bleak time? Definitely. Is it a cause for celebration? I don’t think so.
Is it nice to be able to go to the Kosel? Yes. Does that mean Yom Yerushalayim should be celebrated? I don’t see why it should, given the history and the facts, even if one doesn’t consider this to be further illegal hisgarus baUmos which would mean it’s definitely not a cause for celebration.
Had Zionism not started back in the 1800s, it is quite likely we would have access to worship at the Kosel even without Zionism. I have yet to hear anyone disprove or even suggest a plausible reason that this would not be the case.
Given all the political upheaval of two world wars in the last century, it is overwhelmingly likely that had Zionism not started, (Moshiach would have been here by now and) Eretz Yisrael would have been no different than the UK or any of its former colonies, which is to say Jews would have been free to worship anywhere and to buy the same lands they now own there and to build there whatever they want just as is done in chutz laaretz. Yeshivos, shuls, et al.
That is a definite possibility. Is it certain that this would have happened? No. But is it a definite possibility? Absolutely. Anyone who denies this has either been blinded by Zionism and is not being objective.
So there is no reason to celebrate Yom Yerushalayim or any other Israeli holiday because Zionism was opposed by (most) all gedolim, not just Rabbi Teitelbaum of Satmar, and Zionism is clearly nothing to celebrate (that much is painfully obvious in retrospect) and, therefore, neither are its “bright spots” (assuming it is even an overall bright spot).