Could being overweight cause the pain from spondylolisthesis to be worse?

Yes: Obesity plays a role in back pain in general and now you are adding an unstable level. It may also signify a lack of regular exercise as well which is a factor. Secondary to the obesity may be diabetes which can affect your nerves directly and lead to a neuropathy which may compound any stenosis symptoms if present with the spondylolisthesis level.