Do you get annoyed, after a mini-epic side quest, to end up getting just some run-of-the-mill trinket?
It depends on the game and the presentation. For example, I play Mass Effect primarily for the story and world-building, so I wouldn't feel too bad getting a minor reward for an interesting side quest. However, in games where you have to manage limited resources, it sucks if you end up losing more than you get out of the quest. The reward should feel proportional to the risk you took.
It can be annoying, but as
@Pifanjr said, it really depends on what game you're playing. From an RPG player's perspective (me), some games are more story focused than loot focused, such as the Mass Effect games. It's not about finding some BFG in a chest at the end of a side quest, but how you solve the mission and the reactions or results of your decisions, many of which can be done through conversation choices.
But I know what you mean, I've had some side quests in my current game of The Witcher 3, that when completed I only found some odds & ends crafting materials and a sword that's several levels than my current sword and does much less damage, even though it's labeled a "relic". I'll often say (profanity redacted), "Seriously? That's it?"
I've had the same happen to me in Skyrim & Fallout 4, where the "loot" found at the end of a quest was useless to me. But in those two games crafting my own weapons & armor was more important, and yielded far better equipment than anything I found. For me, gathering crafting resources in those games was far more important than finding a unique piece of equipment. A good crafting mechanic can sometimes mitigate finding subpar loot.
On the other end of the spectrum, there are ARPGs like Diablo 2 & Grimdawn, where your characters success is going to depend upon what kind of loot drops you get (that and how you build your character's skills). Bad loot equals dead character. Just ask
@Colif who is the king of ARPGs, or look at his Grimdawn thread.