What you'll probably find is that the bottom of the radiator is getting plugged w/debris.
Fine chaff always finds a way to work past the two screens and get clogged in between the radiator fins.
You can expect the engine to run alittle hotter when the ambient temp gets into the 90's and higher but the cooling system is designed to properly cool the engine in these hotter temps, as long as it's moving enough air through the heat exchanger (radiator).
Chaff/debris hinders it.
Like has already been mentioned, if you have an air compressor get a long wand air gun that can reachdown in between the fan blades and blow the debris out of the radiator back towards the seat. I don't remove my battery, just the battery compartment screen and the radiator screen, then I usually blow the radiator out every 10 hrs (when I grease the MMM btw).
I can tell when the radiator is starting to clog up by how high the temp gauge shows after an hr or so of finish mowing. Blowing it out every 10 hrs works good for me.
When blowing it out don't use the highest pressure you can as you stand the chance of bending those fins. Set the compressor to about half full open and try it, adjust accordingly...
If one doesn't have an air compressor, a good leaf blower will work but the key is getting at the bottom of the radiator.
I wouldn't recommend a pressure washer for the same reason as using an aircompressor set at it's highest output, bent fins don't cool as well..... But if one uses one as the only resort, use a tip that minmizes the pressure.
If these BX radiators were bigger it wouldn't be so critical but since they are "just" big enough to do the job, keeping them debris free is the key to a happy diesel engine

Dave