Yeah, it's just OK. "The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast....The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech."
So, no magic weapon. No magic armor. No magic items at all in fact. I am unsure if it can use feats but it won't matter much as most of those feats won't apply very well to "fist" and "rock" (none of Great Weapon Master, Sharpshooter, Shield Master, Polearm Master, etc.. would apply). Can it use Battle Master manuevers? Unsure. But, I am pretty sure yes, the damage output of your typical 7th level fighter will be more than "Multi-attack: Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage." You were likely at or beyond +9 to attack already (+5 ability bonus, +3 proficiency, +1 magic item, none of which counts fighting styles like Archery which would put it to +11). You already had two regular attacks, you likely had something giving you a bonus attack, and you may well have reaction attacks as well. Your weapon is already doing pretty good damage, modified by something or several somethings (Great Weapon master, Sharp Shooter, Battle Master manuevers, magic, etc..). I think your typical fighter is in fact dishing out around this damage or more...but with a MUCh better armor classs, and far more flexibility and use of their feats (they have two or three of them at this stage) and class options and magic items. In fact as a non-humanoid you're not even a valid target for some beneficial spells from your spellcasters I believe while in Giant Ape form.
Naw, I think friendly Polymorph is best used when the fighter is already at risk of going down...not to start a battle. It's not as effective, it's not as flexible, and it's not as fun.