I am arriving at Gare de Lyon from Zurich connecting to Eurostar at Gare du Nord for London, is 90 minutes enough time to make that connection?
90 minutes should be plenty if you're simply transferring from Gare de Lyon to Gare du Nord.
The RER D train line connects Gare de Lyon and Gare du Nord which are only 2 stops apart and about 11 or 12 minutes train ride in total. There will be 4-5 minutes of walking between the surface level intercity trains down to RER though, so keep that in mind. Then up to 5 minutes waiting time on the platform mid-day. So about 25-27 minutes maximum total "transfer travel time" between Gare de Lyon and Gare du Nord.
You'll still need to buy a regular Metro ticket (ticket t+) to make this transfer, from either automated ticket machines or the RER D ticket window ahead of its fare gates at Gare de Lyon.
When making your way to the RER D trains, after passing through fare gares, you'll need to choose the right platform / direction which can be any of these: Creil / Goussainville / Stade de France Saint-Denis (perhaps others depending on rail works and interruptions). Trains in this direction will all stop at Gare du Nord on their way north. You do not want directions: Malesherbes / Melun / Corbeil Essones / Juvisy. Those RER D trains are heading in the opposite direction (south).
There should be overhead panels on the platform showing you which stations will be served by the next train to arrive. Double check with those that you are on the correct platform / heading in the right direction.
When arriving at Gare du Nord follow the signs for "Grandes Lignes" / Main line trains, which are the surface level trains including Eurostar. You'll need to check-in for Eurostar one level above surface level using the stairs / escalator in the middle of the station. See the Gare du Nord / Eurostar article for photos to orient yourself and know what to look for to find Eurostar check-in area.
You should be fine.