This doesn't seem likely for several reasons. Most hospital waste is considered low level waste (LLW) and not very hazardous and would make for a anticlimactic dirty bomb. In fact LLW is usually just incinerated or buried with only minimum precaution. Let's assume they got a hold of something that was not LLW waste and was a high level waste (HLW) item, like in Goiania.
Something that would actually work in dirty bomb would have to be stored in a shielded container, thus you wouldn't need bananas anyway as bananas radiate very little in comparison to almost everything else (see below). But if the crooks were as ignorant about radiation as the scrap metal recyclers in Goiania who spread the glowing powder and dust around and even let a kid rub it on her skin for effect, then perhaps they took it out of the container to transport it.
At this point the exposure levels would be so high that bananas wouldn't even compare. So other than providing a snack in between bouts of vomiting from radiation sickness I don't see what bananas would do to mask it. Banana radiation does make a nice little plot trivia that helps the story seem more realistic.
Let's compare some numbers in banana terms (BED banana equivalent dose):
80 million BED = Fatal dose even with treatment
20 million BED = Severe radiation poisoning, fatal in some cases
500,000 BED = Maximum legal yearly dose for a US radiation worker
40,000 BED = Ten years of normal background dose, 85% of which is from
4000 BED = 1 Mammogram
1000 BED = Approximate total dose received at Fukushima Town Hall in
two weeks following accident
400 BED = 1 Flight from London to New York
300 BED = Yearly release target for a nuclear power plant
200 BED = Chest X-ray
50 BED = Dental X-ray
1 BED = Eating a banana ( 0.1 μSv = 0.0001 mSv dosage)
That's a lot of bananas to get close to a dosage that would be fatal, so it would take a lot to cover up an unshielded radiation source. In fact our bodies carry around more BED doses than a banana.
We surrounded by naturally-radioactive materials, and are constantly bathed in radiation originating in the rocks and soil, building materials, the sky (space), food and one another. A typical background level of exposure is 2-3 millisieverts per year (mSv/y) or 20000 to 30000 BED / year.
Bananas probably wouldn't provide a lot of cover. After all don't they scan a lot of organic products and know how much to expect? So anything additional would be suspect and HLW would stand out unless it was so well shielded that there would be no point for bananas anyway.