Is that really an Excel file (.xls, ...) or is it a CSV file? (.csv, .tsv)
Either way, it is a poor format that's for sure.
If it is CSV, then use the csv node, skip the first line & make the next row the column headers.
I've not used the spreadsheet-in node so I don't know whether that has any way to do something similar but I'm guessing not. So short of walking the whole JSON object to change the column names yourself (or wrangling JSONata to do it for you), I think you are rather stuck.
If this is Excel and you are using a reasonably up-to-date version, you will likely find it easier to manipulate the origin data using PowerQuery to get more sensible output.