Salam alaikum. All over the world we can see mosques that were built upon graves of pious people. Let us see, what shia imams said about praying over graves.

عن أبي جعفر عليه السلام قال: إن رسول الله ص قال: لا تتخذوا قبري قبلة ولا مسجدًا، فإن الله عز وجل لعن الذين اتخذوا قبور أنبيائهم مساجد
“From Abu Jafar (alayhi salam): Prophet (sallalahu alayhi wa ala alihi wa salaam) said: “Don’t take my grave as a qibla or as a mosque, indeed Allah cursed those who took graves of prophets as mosques”.

I should notice that mosque could also mean a place where doing a lot of sajda. i.e place where a lot of worshiping.

ن أمير المؤمنين عليه السلام قال: بعثني رسول الله ص إلى المدينة، فقال: لا تدع صورةً إلا محوتها، ولا قبرًا إلا سويته، ولا كلبًا إلا قتلته
“Ameralmomeneen (alayhi salam) said: Prophet (sallalahu alayhi wa ala alihi wa salaam) send me with order “Do not leave an image without obliterating it, or a high grave without levelling it, and dog without killing it”.

قال النبي صلى الله عليه وسلم لا تتخذوا قبري قبلة ولا مسجداً؛ فإن الله عز وجل لعن اليهود حين اتخذوا قبور أنبيائهم مساجد
“The prophet (sallalahu alayhi wa ala alihi wa salam) said: “Don’t take my grave like place of worship or “Kibla”, Allah cursed jews which build Mosques over graves of their prophets”.

رَسُولَ اللَّهِ ( ص ) يَقُولُ لَا تَتَّخِذُوا قَبْرِي عِيداً وَ لَا تَتَّخِذُوا قُبُورَكُمْ مَسَاجِدَكُمْ وَ لَا بُيُوتَكُمْ قُبُوراً
“Prophet (sallalahu alayhi wa ala alihi wa salam) said:”Don’t make feast place on my grave, and don’t make Mosques from your graves, and graves from your houses”