Do you think her acting abilities were limited or was she just lazy and didn't want to step out of her romantic comedy comfort zone (especially in the 1960s)?

https://s9.postimg.org/uxa6491wf/image.gif

It may not be either case you suggested. Perhaps she only played the types of roles that she wanted to.

Miss Day HAD played genuinely dramatic roles with a good amount of depth in the 1950's. Let's not forget that she debuted her iconic theme song "Que Sera Sera" as the mother of a kidnapped child in Alfred Hitchcock's "The Man Who Knew Too Much" (and the song itself became a vital plot point)!

I think by the time TDDS debuted, Miss Day was already getting burned out with performing in general and only did the show as a means of honoring that contract her late husband signed without her knowledge AND as a means to pay off the enormous debts he'd incurred via bad investments. Yes, she did a great job under these rather trying circumstances but, once she became solvent again, she only wanted to focus on her passion for making sure animals are taken good care of and I think that's helped her live to an impressive age as a pretty old lady despite the many setbacks and tragedies she's had to endure.

And let us not forget 1960's wonderfully dramatic movie "Midnight Lace". She was very good in that totally dramatic role. In fact, she gave one indication of why she didn't act in movies like that anymore once it was completed. She said it was so emotionally taxing for her that she just didn't want to go through that again. Also, remember that from 1959 to 1964 she was the number one box office actress in the country doing light comedies. If you're number one, she must have figured why mess with the formula.

Apparently the OP never saw Love Me Or Leave Me.