Can I just use a standard UPS in an outdoor NEMA 3R cabinet and call it an outdoor UPS?

No, because several problems will occur. First of all, the batteries in standard indoor UPS are rated for 0 to +40 degrees only. Also, the indoor UPS does not use a temperature-compensated battery charger. As a result, the battery lifetime will be only a few weeks (or few months), especially during the summer when temperature inside an outdoor cabinet can easily exceed +40 degrees C.
The VRLA (valve-regulated lead acid) AGM (absorbed glass mat) battery will start out-gassing and the electrolyte (battery acid) will be quickly depleted when the wrong battery charging voltage is used, which occurs when battery temperature is over +40 °C. With a loss of battery acid occurring in a just few hours or days, the glass mats become dry and current can no longer flow between the battery plates.
A fine mist of battery acid (similar to mist produced by a bug spray) will shoot out of the battery and cover everything inside the cabinet This causes severe corrosion that will destroy everything inside the cabinet, including the cabinet itself. Also, unsuspecting operators can rub their eyes with fingers after touching parts inside the cabinet and receive severe eye injury.
In conclusion, answer is an emphatic NO.