My personal opinion:
There was no real need to convert the Hagia Sophia into a mosque. By no real need I mean: there are plenty of mosques in Istanbul and even near by Hagia Sophia.
BUT:
Turkey is a sovereign country. Euros must understand that they cannot demand anything from them if they do what a sovereign country does: acting like a sovereign country.
Europe is full of ancient mosques and they converted some into museums, churches and the Greeks went even farer by converting them into discos and night clubs. So? Their country, their rule.
The discussion is a very toxic one because Euros are mostly atheists or agnostics belittling religious sentiments all the time. France has an extreme Anti-Muslim policy, Switzerland bans minarets, in Germany you have also stupid discussion about everything slightly Islamic. But if they can critisize another sovereign country, everything is good enough to do so.
Especially Germans have a problem when Turks act sovereign. Because they think it's still the 1960ies and the Turks will be submissive to them. This Yes-Boss-Mentality is not anymore. But some Germans think Turks should ask them first if they do something in their OWN COUNTRY.
The German embassy in Pakistan comments on Pakistans internal matters on their official Facebook page. They demand abolish the death penalty, be more gay friendly, do this do that.
If the Turkish or Pakistani ambassador in Berlin would do the same, there would be a big media outrage against them.
"Terrorists say abolish freedom" would be the headline of the yellow papers.