Would it be smart for the Tyrell’s to contribute to the rebuilding of the Riverlands, so as to garner support from the smallfolk and Lords of the region in a possible Lannister-Tyrell conflict that seems to be brewing, or would it be prohibitively expensive/ineffective PR?

If they’ve got money to spare, certainly. Especially with House Tarly’s new connection to Maidenpool, there’s a beach-head there. 

If I were them, I’d focus my efforts at the area around Stony Sept, Pinkmaiden, Acorn Hall, etc. which are reasonable close to the borders of the Reach and easier to incorporate.