In my opinion, several factors should be considered.
First of all, how efficient is solar energy? Do the lights burn brightly, as is required of Christmas lights? Most solar lamps used as landscape and yard lighting is too dim. Back in the 1980s, energy saving bulbs were becoming the vogue. They could be seen to the naked eye but were far too dim to be photographed or filmed. They didn’t cast much light,
Second is the spectrum. In landscape lighting, we get amber coloured lighting and the old black-and-white-teevee spectrum and colourless lighting. Christmas lights need to be of a warmer spectrum.
And third, how efficiently do they charge? Winter days are more likely to be cold and grey and winter nights are long. Could they get an adequate charge, especially on those white Christmas days?
In my opinion, solar lights are like fat-free eggnog. It might be a good thing but defeats the purpose of warm and coloured lights on a winter night.