@JaneJeffer
Yes, you can hide whole topics. If you're using the app tap on the three horizontal lines at the top right of the screen, then tap on MyMumsnet, and at the bottom of the screen click on 'Open additional MyMumsnet settings' - this takes you to the web version of the site, where you'll be able to click on 'More settings' then 'Talk settings', then scroll down to 'Manage hidden threads and topics' - once there you can choose which topics you'd like to hide.
Hope that all makes sense!