Try using "wrap-table-header"

Specifies how the table header is wrapped. Set to 'all' to wrap all column headers. Set to 'none' to clip all column headers. Set to 'by-column' to wrap/clip column headers based on the wrap/clip setting for that individual column. The default is 'none'.

https://developer.salesforce.com/docs/component-library/bundle/lightning-datatable/specification

Thanks You,
Preeti Pujari,
Salesforce Consultant, Winobell Inc
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