In short: Yes.
Quark had a major setback in the past which left him broke
QUARK: Listen, son. When I was a young man, no older than you, I had an apprentice position with the District Sub-Nagus. I licked his boots like you couldn't believe. He loved me. I was his golden boy. I was on the high road to the top of the Ferengi business world, and then it all fell apart.
ARJIN: How?
QUARK: Rule of Acquisition one twelve. Never have sex with the boss's sister. I was fired, broke. It was quite a setback to my ambitions.
ARJIN: How'd you recover?
QUARK: Never did. Look at me. Tending bar out here in Wormhole Junction while the big boys fly past me at warp speed. You only get one shot at the latinum stairway. If you miss it, you miss it. Welcome to the club, son.
(DS9: Playing God)
Quark runs his business in a way that does not always bring him a maximum of profit
This is one of the key accusations made by Brunt in the episode in question - that Quark does not all he can to maximize his profit:
QUARK: Why? What have I ever done to you?
BRUNT: Done to me? And you call your brother an idiot? Nothing you've ever done to me has been more than a minor inconvenience. No. Protecting your mother from an FCA audit, and secretly settling with your striking employees were nothing more than symptoms of a vile and insidious weakness. A weakness that makes me loath you, not for what you've done but for who you are, what you are.
QUARK: A bartender?
BRUNT: A philanthropist.
QUARK: I am not!
BRUNT: You give your customers credit at the bar. You only take a thirty percent kickback from your employees' tips, and you sold food and medicine to Bajoran refugees at cost!
(DS9: Body Parts)
Similarily, he did not take an opportunity that really would have extended his profit. He loaned his cousin Gaila latinum, and in return, he was promised a shuttle. Gaila invested in weapons and advised Quark to do the same - he didn't, and while Gaila earned enough to buy himself a moon, Quark was less fortunate:
QUARK: I should've listened to my cousin Gaila. He said to me, Quark, I've got one word for you. Weapons. No one ever went broke selling weapons. But did I take his advice? No. And why not? Because I'm a people person. I like interacting with my customers. Like you and I are doing right now. Talking to each other, getting to know one another
GARAK: I can see the attraction for you.
QUARK: But when you're dealing in weapons, buyers aren't interested in casual conversation. They just want their merchandise, no questions asked. It's so impersonal.
GARAK: Your charms would be wasted.
QUARK: Exactly. So now Gaila owns his own moon, and I'm staring into the abyss. And the worst part is, my only hope for salvation is the Federation.
(DS9: The Way of the Warrior)
This incident really bugs him, he even tells Odo about it:
QUARK: No. He warned me never to leave home. He said there were plenty of business opportunities right outside my door. But no, I had to follow the seventy fifth Rule of Acquisition. Home is where the heart is but the stars are made of latinum. A lifetime of scheming and plotting, wheeling and dealing, and what has it got me? One measly little bar. My uncle Frin owns thirty and my cousin Gaila...
ODO: I know. He owns a moon.
(DS9: Civil Defense)
There are occasions where he actually has no liquid assets
This is referenced in the quote above, and it's a major plot point in DS9: "Bar Association" :
QUARK: I have an announcement to make. I've just been going over today's receipts and it's not good. The only thing this Bajoran Cleansing Ritual has cleansed is my profit margin. So, starting tomorrow, everyone's salary gets cut by a third.
ROM: No, no, it's not fair. I'll talk to him.
LEETA: It's ridiculous.
QUARK: It was either that or fire half the staff. Don't bother thanking me.
LEETA: I can't afford a pay cut.
QUARK: It's either a pay cut or a layoff. You decide. All right everyone, get back to work. I want this place cleaned up and the lights off in ten minutes.
(DS9: Bar Association)
(It's up to you whether or not you believe him, in my opinion the plot would make a lot less sense if Quark would actually have the money and therefore the opportunity to end the strike)
Other Ferengi think that he is not very successful
Other than Brunt, we see other Ferengi calling him "lobeless" (as in, not having the lobes for Business)
ZEK: No, my mind's made up. I've already chosen my successor. A leader whose vision, imagination, and avarice will oversee Ferengi ventures in the Gamma Quadrant. The new Grand Nagus. Quark!
(General disbelief and consternation)
GRAL: He's a lobeless amateur!
KRAX: Father, have you lost your mind?
NAVA: I won't stand for this!
(The business men storm out, overturning chairs)
(DS9: The Nagus)
