On face value, leaving aside Olenna's agendas, the basic idea is: don't try to be too clever, because clever plans never go as planned. You've got power, use it.
There's an important detail missing from your quote: before she said about being a dragon, she said something like, "Most lords are sheep". Then she said Dany is a dragon in contrast with the sheep. It's the old idea that is a theme throughout the show that power resides where people believe it resides - her spin on it is, if you want to herd the sheep, you don't come up with some terribly clever experimental sheep influencing system, you just assert yourself over the sheep.
If you want lords to fall in line, and you have dragons plus a fearsome army, don't attempt some clever but delicate plan which will fall apart if some unforeseen problem emerges (like, to give a purely hypothetical example, if one of your fleets is intercepted and set on fire by a strong naval ally you didn't realise your enemies had).
Just get on with asserting yourself the old fashioned way by terrifying them with your three dragons.
Of course she also has an interest in reducing Dany's trust in her rival advisors. She saw Tyrion as a fool ever since he begged her to pay for the royal wedding, she never trusted Varys, and dislikes the Dornish who are traditionally Tyrell enemies. I think we don't know her views on the Iron Islanders (beyond liking the phrase "We do not sow") but given her general attitude they're probably unrepeatable.
She doesn't want these other oafs with their foolish plans getting in the way of her revenge. At this point, with her entire family dead, she just wants to watch the world burn.
To give a little more context to her contempt for the clever men she outlived, think of a few:
- Her "oafish" son, who thought he was clever with his timely allegiance with the Lannisters, which worked for a time then backfired because (as Olenna had feared) he couldn't control it through all the unforeseen twists and turns
- Loras and Renly, who had a clever plan for seizing the throne, which failed because of unforeseen events they couldn't respond to
- Tywin, who cleverly dominated Westeros for decades, but in doing so stacked up enemies, even in his own family, who eventually killed him in unforeseen circumstances
- Her late husband, who she talks about as a symbol of all the many foolish oafs she is always despairing of, who was so busy observing the movements of birds while hunting he didn't see the cliff edge he galloped over
Olenna's schemes, in contrast, tend to be pretty blunt and therefore less fragile in the face of the unexpected. Just buy influence with grain, wealth and marriages instead of fighting or scheming for it. Just get on and poison the guy who's a problem, letting someone else sort the details and someone else take the blame. Just burn the bloody queen who killed your family.
