You’re a character, and you know flaws personally. I leave the topic to you
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If it’s a young (1-3 yo) Riesling with very little else besides a huge smack of diesel that seems to be completely out of balance, then yes, that can be considered a flaw.
If it’s an aged Riesling with a lovely streak of petrolly diesel character that intermingles with the fruit and mineral nuances, it’s not a flaw by any standards; it belongs there.
However, Riesling is in interesting variety in the sense that it calls for not only direct sunlight, but also certain compounds from the soil to be able to synthesize TDN. Riesling grown on certain soils will never develop any petrol characteristics, no matter how long it is aged.
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I can remember a thread about this long ago. Either on Eboob or the early days here. Someone asserted that the ‘petrol’ note was a sign incomplete ripeness. I feel like its a matter of opinion.
It seems that if you always make sure Riesling gets ripe enough that there is never a petrol note, then its likely to take on the more fruit forward and tropical characters that will push out the mineral and cooler fruit notes so many love about classic Riesling.
This discussion most reminds me of pyrazines rather than VA or brett. I also remember a discussion where the Chairman himself, Mark Squires declared that any amount of pyrazine was a flaw due to insufficient ripening. Bollocks.
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Actually from what I’ve understood, the concentration of TDN precursors increase as the grapes ripen. Their development is also suppressed in cooler temperatures. A wine made in a warm vintage and fully ripe grapes will have more TDN precursors than a wine made in a cool vintage with less ripe grapes. However, I’ve understood that the effects of these are much smaller compared to the effect of direct sunlight to grape bunches, which promotes the development of TDN precursors more than anything else.
However, it is true that a Riesling made with very ripe grapes might have more pronounced tropical fruit flavors which are much more effective at suppressing the TDN aromas compared to the lighter, less heady aromas in wines made with not fully ripe grapes, which might make it feel that Rieslings made with fully ripe grapes wouldn’t be petrolly as often!
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There have been a couple of studies, and I think. (IIRC) that it does relate to sunlight on the bunches, not ripeness.
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Some people say there is no such thing as a stupid question …
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Within the last week (?) someone on here posted this: The Science of Petrol Aromas in Wine | SevenFifty Daily
Which I found to be educational on this topic.
But no, it’s a beautiful component that, when +/- balance is wonderful.
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In fact, if your Riesling does not have petrol notes, it’s highly advised that the Riesling is transported to a gas station for a quick double decant.
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I don’t believe its a flaw, but I dont care for it either. The 2015 GH Auslese I just opened was clean as a whistle!
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As opposed to creating a thread about broken bottles?
No doubt! That one is pretty dumb indeed!
Petrol is also common in Grenache Blancs. My guess is that the skins also contain TDN but no study has clever been done. Every GB I’ve ever made has varying levels of petrol . . .
Cheers
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If you can’t get at least 30 mpg is it even a Riesling?
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When it’s not there I become sad 
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I personally love the hint of petrol in a good Riesling, but I also like the funk in Bionic Wines too. Both can be polarizing depending on what ones palate appreciates.
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The fascinating thing to me is when someone who is not used to smell encounters it - and is ‘scared’ to say what they smell because, well, how in the heck should petrol be in a wine? 
Cheers
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Petrol is often caused by grapes beeing exposed directly to sun and its higher in hot vintages. When Riesling then matures its developing the TDN which causes the petrol note. I think it’s safe to say that Petrol is not something many producers are looking for to be found especially in a young Riesling. I talked about this with Clemens Busch in a masterclass and he said for him too much petrol its something like an error, he tries to avoid it. There are people who appreciate it and look for it and others who can‘t stand it, its a matter of taste. Ripe Rieslings from colder vintages typically have less petrol than Rieslings from hotter vintages. Personally I don‘t mind petrol but i tend to prefer colder vintages.
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Key for me is the 1st word in the OP title. SHOULD petrol be considered a flaw?
When stated like this then the answer is no. Petrol notes in Riesling are feature that some folks like and others dislike.
Like @Cris_Whetstone stated it is more like Pyrazine. Some dislike it and some like it.
But it is not flaw like TCA or premox.
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I posted this in the What Riesling are you drinking? thread, but it should be here as well
“Petrol” is very clearly a characteristic of the riesling grape, and something that develops more or less strongly in the vineyard, depending on growing conditions. According to the article, more sun, water stress, nitrogen deficiency, are all factors that increase the amount of the precursors to the “offending” compound TDN (or, more completely, 1,1,6-Trimethyl-1,2-dihydronaphthalene).
Given that there will always be at least some TDN in Riesling grapes - and not something “added” by the winemaker, or allowed to creep in through vinification choices - I don’t see how it can be called a flaw. How much it gets expressed may be controllable to a degree through vineyard practices, but that control is probably limited.
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