"There is nothing wrong with reading from the Mus-haf whilst lying down.
Al-Bukhaari (7549) and Muslim (301) narrated that the Prophet (peace and blessings of Allaah be upon him) used to lie in ‘Aa’ishah’s lap and recite Qur’aan.
Al-Nawawi (may Allaah have mercy on him) said in Sharh Saheeh Muslim:
I was just concerned because once my MIL forbade me because according to her . Husband and wife sleep on bed n they do make love so it's not good to recite quran on bed
I was just concerned because once my MIL forbade me because according to her . Husband and wife sleep on bed n they do make love so it's not good to recite quran on bed
What your MIL said is based on cultural etiquette. In my place recite Qur'an while sitting on bed is okay. But recite (not just reading) Qur'an while lying on bed, on sofa, on carpet, considered as less-appropriate.
My advice. If your MIL told you to not recite Qur'an on bed, just choose to recite Qur'an on chair or on carpet. No need to argue her. Try to respect cultural etiquette.