Should I be worried about this Pumpkin plant discolouration? see pictures:
It's possible there is some magnesium deficiency here. Lots of information online including pictures of plants with magnesium problems. When doing a search make sure to specify "magnesium deficiency plants" otherwise the search engine will think you mean deficiency in diet. There are also recommendations for a quick remedial solution involving a foliar spray of Epsom salts which might just get your plant going again. Follow the instructions closely.
Also having grass that close will be competition for the pumpkin, which will have a hard time preventing the grass from stealing the nourishment.