On screen rendition of PDF at anything other than 100% view may not be good. How does it print?
Make a PDF from the Excel sheet first and place that instead of a PNG screen shot.
Or import the XLSX file directly and format it in InDesign.
Why are you using the png anyway?
Should I be using something different like jpeg?
Paint says that png is the best quality for files - if you know something better, would love to know!
Just trying out your first suggestion - will get back to you if it works
PDf/X-4 is the best when text or drawings are used.