Will users think my UI looks unattractive?

This depends on the situation; it's not a one-size-fits-all answer.

Scenario 1: If your product is a tool, especially one that solves a specific pain point.

Frankly, whether users will use your product hinges on whether it can help them solve problems and improve efficiency. If your tool is a "unique skill" or 10 times better than others on the market, then a slightly ugly UI really isn't that important.

Think about many tools used by programmers, some scientific research software, or even internal systems for governments and banks – their UIs aren't exactly pretty, but people still have to use them because they get specific jobs done. In such cases, the baseline for UI is "usability" not "beauty". As long as the functional flow is clear, users don't get lost, and there are no "anti-human" operations, most users can tolerate it. They're there to get a task done, and once it's done, they leave.

Scenario 2: If your product is for the general public, such as social, shopping, or lifestyle apps.

Then UI becomes very important. For these types of products, user experience itself is part of the core product value. An ugly interface will make users feel your product is "shoddy," unprofessional, and lacking trustworthiness. There are too many similar products on the market now, and the cost for users to switch is extremely low; a bad first impression means a swipe-to-uninstall in an instant. A beautiful, smooth interface can make users feel pleasant and more willing to spend time exploring it.

My advice to you:

As engineers, we don't necessarily need to become design masters, but we can take shortcuts to ensure the UI doesn't "hold us back."

1. Don't reinvent the wheel: Directly use mature front-end UI frameworks like Ant Design, Element UI, Bootstrap, Material-UI, etc. These frameworks provide a complete set of design specifications and ready-made components. You just need to combine them like building blocks to create a professional and modern-looking interface. This ensures your UI is at least above average.
2. Learn from others: Look at well-made products in your field: how they are laid out, where buttons are placed, and how workflows are designed. Imitation is the best way to learn. This isn't about plagiarizing code, but rather drawing lessons from their market-validated user experience designs.
3. Focus on flow, not aesthetics: In the early stages, you should focus more on "can the user successfully complete what they want to do?" rather than "does this button's color look good?" Find a few friends or target users, let them try it out. Don't ask them "does it look good?", but instead ask them to complete a specific task, like "try publishing an article." Then observe them to see where they get stuck or hesitate – that's where you most need to make changes.

To summarize:

Users might not reject you just because your product is "ugly," but they are very likely to abandon you due to "difficulty of use" and a "lack of professionalism." First, use UI frameworks to ensure a basic level of professionalism and usability, then focus your energy on refining core features and workflows. This is the right path in the early stages of a startup.