Lisieux

You can drive it but it would not be convenient by train to go from Lisieux to Giverny.
Simply if you have another day in Paris take Giverny separate day trip

I went by train from Paris maybe 15 years ago and don't remember any difficulties, but ttransport may have changed since then.
Before I went, I listened to Kathryn Harrison's Saint Therese of Lisieux: Penguin Lives Series. Reviews vary but I found the book informative. Harrison is a troubled author, true, so a judicious read is necessary.

Getting to Lisieux from Paris by train is pretty straightforward. However getting from Lisieux to Giverny is not as easy because there's no direct route, you need to connect through Rouen which adds a bit of time. It could be done, but it would be a pretty long day.

Whether that is viable depends on three things, I think:

• How early are you willing to start out from Paris? 7:06? 7:41? 8:44? After that, it would be hopeless, I think. Lisieux is 1-1/2 to 1-3/4 hours from Paris.
• How much time will you want in Lisieux?
• How much time will you want in Giverny?
  

I think you need to take a bus from the nearest rail station (Vernon) to Giverny, and I don't know how long the bus takes or how often it runs. To travel from Liseux to Vernon you'll have to change trains at least once, and the travel time will be between 2 hours and 3-1/2 hours.

Best case, the train back from Vernon will take 48 minutes.

You'd need to really study the train schedules (see Deutsche Bahn website) and research the bus arrangements for reaching Giverny, then pad the schedule a lot for safety. Don't forget that you'll need to eat at some point.

Do you have plans to visit a particular sight at Lisieux, or do you just want to walk around? If the latter, I'd start my day at Giverny, then head to Lisieux, which you would still be able to enjoy late in the afternoon. They'll throw you out of Monet's garden at 6 PM if not a bit earlier.