Rashi in Parshas Ki Sisa, Shemos 34:3, says the following:
I simply don't understand this Tanchuma. I have nothing to propose here. The best I can do is to try to ask a she'ailas chacham.
1. What does Ayin Hara have to do with the sin of the Egel? Does the Tanchuma mean that when the Sattan showed Klal Yisrael a mirage of Moshe Rabbeinu's bier, he was motivated by a jealousy that stemmed from the marvels of Mattan Torah? Or that the Ayin Hara caused the Eirev Rav to demand an Eigel? Or that if not for the Ayin Hara, they couldn't have been broken even if thrown down?
2. We do find a concept of Tznius in doing mitzvos. The example that I always use is the Gemara in Zevachim 59b. There is a machlokes Tana'im about how tall the Mizbe'ach in the Mishkan was. Rebbi Yehuda says it was three amos tall, and the curtains were five, allowing you to see the Kohen, but not the avoda in his hands. Reb Yosi says it was ten amos high. The Gemara asks, but if it was ten amos high, everyone could see the avoda! This is impossible!
3. On the other hand, we find the din of ברוב עם הדרת מלך, that it is a Hiddur, or a Kiddush Hashem, to have many onlookers when you do a mitzva. You might be thinking, as I initially did, that Rov Am only applies when everyone present is participating in the mitzva, not merely looking. You might be right. But almost all the achronim and poskim, with a great deal of support from Chazal (Yoma 70a, Machatzis Hashekel 147 SK 11,) hold that the din of Rov Am applies even when one person is doing the mitzva and the others are merely looking. As Reb Heshy Lichtman pointed out to me, if Chillul Hashem is exacerbated by witnesses to an aveira, kal vachomer Kiddush Hashem is enhanced by witnesses to a mitzva. The raya from Yoma is very debatable. Heshy's raya is solid.
So, how does this Tanchuma accord with the din of Rov Am? If Rov Am is a Hiddur and a Kiddush Hashem, how can it possibly create a danger of Ayin Hara? And if the two are not mutually exclusive, which is the preferred path?
4. When the Tanchuma uses the words Ayin Hara, does it mean Reb Yitzchak or D'bei Reb Yishmael in Bava Metzia 42a:
A Chkira. What is the primary cause of the damage of Ayin Hara: pride or envy? Is Ayin Hara caused by the other person's envy, or is it a consequence of pride on the part of the victim? Are we missing the problem of Ayin Hara by focusing on the jealousy, when we should be looking at the pride that makes a person vulnerable to jealousy? Maybe Ayin Hara happens when a person is not only happy that he has something, but when he's happy that "I have it AND YOU DON'T." If that is the case, maybe what the Tanchuma means is that we felt pride that everyone else knew we had a unique and personal relationship with the Ribono shel Olam, and it was that pride that set us up for the Chet Ha'Eigel? If that's the case, then maybe the result of the תשואות וקולות וקהלות was פירסום, and the result of the פירסום was pride, and as a result of this pride, the connection of the Luchos to Klal Yisrael was attenuated.
I saw the פרדס יוסף in כי תשא brings down that when Reb Meir Shapiro opened his yeshiva, he made a big showy event, and someone asked him how what he was doing shtimmed with the Tanchuma, and he answered that he was aware of the risk, but the Klal needed to be shaken awake (אויפטרייסלונג) by such an event. I've seen the Rav Kook was asked about this when he inaugurated his building as well, and he answered something along the lines of a distinction between publicizing a concept and publicizing an action (הקול והפרסום לצורך הכרזתם של האידיאלים הם יפים מאד, אבל מצד המעשים יותר יפה היא הצניעות, that sound is good for Besamim but bad for wine in Menachos 87, as I've written about here.). I also saw the the Shem MiShmuel addresses what exactly Ayin Hara means in this Tanchuma, but I don't understand what he is saying. Bottom line: I'm still looking for a pshat that I understand.