Hello. Hope you are doing well.
Mouth ulcers could be having varying causes how ever the most common could be aphthous ulcers.
Next Steps
Lookout for any foods causing flaring and avoid those
Any sharp tooth, get it checked
Any bowel trouble ( constipation ), take care of that
For ulcers per se, yes
vitamin C would definitely help but do take orasore mouth gel half a hour before food thrice a day and multivitamins as well. Also keep up hydrations and lots of green leafy vegetables in diet
Health Tips
Avoid hot and spicy foods
If it doesn’t improve in three days get a physical consultation with ENT