If you want a table service meal during a typical mealtime (not lunch at 3pm or dinner at 9pm or whatever other odd times are left over), yes, reservations are recommended. Obviously, mobile-ordering from counter service locations (many of which have air-conditioned indoor seating) requires little-to-no advanced planning, although it's possible that the available time slots might be 30 or more minutes out from when you place the order.
(*Note that if you're a Landry's Select Club member looking for a table service meal, you have a good chance of being able to get a walkup at T-REX, Yak & Yeti, or either of the Rainforest Cafe locations.)
We were there last week during very low crowds (crowd levels of 1-3 out of 10), and we still had difficulty trying to get any table service reservations last-minute, especially in the parks. If you do want to make table service reservations after you arrive on property, the best time to do so is 24-36 hours before you want to eat, as that's when people will be canceling preexisting reservations to avoid paying a penalty.