This is simply an 'Ask your DM' scenario.
If the DM intends that the motion sickness may have an impact on combat, etc then they may say no, but if it's just a character building, social interaction then I think most DM's wouldn't object.
Note that I'm basing my answer on 'real world' knowledge here. Making a tea is not difficult and medieval people would all have had a basic knowledge of simple things like 'nettle' or 'pine' tea (literally, stinging nettles in hot water!)
I would ask the DM in the following way:
- Is it possible my character has knowledge which could help alleviate the motion sickness?
- Can I now try and make the tea? (note this is tea, not a potion, it's pretty simple)
If possible discuss with them out of session that you want to do this, maybe it then becomes part of your backstory (for example "I was often sick as a child but my mother used to make the most delicious teas for me to drink"). You could even use this as an opportunity to diversify your skills, maybe start learning some herbalism skills.
I'd recommend you have a look at sources like 'Dump Stat Adventures'. They have some useful skills expansions which develop things like herbalism, chefs tools and so on. If your DM is willing, these could prove useful.
Above all, discuss with your DM and don't be upset if they say 'no'.