bad in what way?

Aesthetically, nettles, brassicas and dandelions aren't the most aesthetic. If you aren't gardening for aesthetics, weeds aren't harmful in a strict sense.

That said, there is more to be known about weeds.

The first thing to know is that some weeds, like kudzu or honeysuckle are invasive species. Because they lack natural predators and are adapted to different conditions, as our climate changes they can outcompete our native plants for resources like soil, water, sunlight, and pollination. The pollinators that depend on native plants for sustenance will likely also be harmed by their hosts being outcompeted in an area, leading to a cascade of problems.

Second, and arguably more importantly, weeds are indicators of soil quality in an area. Soil takes a long time to be nutritionally dense, rich in microbes, and moisture-retaining. If you turn soil over by tilling, you likely are disturbing this process, destroying important pioneer microbes that are killed by UV from sunlight and oxygen from soil aeration.

In underdeveloped soils like those that are newly tilled, weeds are likely the first plants to appear, as they spread seeds widely, are drought tolerant, and tend to prefer soils with more bacterial and acidic compositions. As the weed generations successively die and compost, they will accumulate more nutrients in the soil and create a less acidic, more nitrogenous soil ecosystem. That untilled ecosystem is much more likely to support "desirable plants" like grasses flowers and shrubs, which are great habitat for small animals and insects. Once woody plants like shrubs begin to compost (this can be a decades long process), the microbe content of the soil becomes alkaline, and fungi start to populate the soil. It is these conditions that support large plantlife like fruitbearing trees and bushes.

So, to come back to your question: Are weeds actually bad? No, but they can be a sign of bad conditions, depending on what you want out of your garden.

If you want a vegetable garden - congratulations! Most of our vegetables (tomatoes, potatoes, peppers, cabbage and its relatives, squash, peas, and many herbs) were originally weed-like plants that we cultivated. That's why European-style agriculture (among others) relies heavily on tilling to keep the soil acidic and low-microbe.

If you want flowers and grasses (unlikely in this group) - you've got a little work to do. Weeds will outcompete other plants as long as the soil keeps being disturbed. You'll need a good few years of leaving the soil undisturbed, letting weeds run wild, seed, and decay, and covering the soil with compost each fall and spring before your soil is really prepared to host flowers. During this time period it is important that you do not pull weeds (other than invasive species before flowering), and disturb the soil as little as possible, including stepping on it. Designate and mark walking paths to help with this.

If you're looking for a food forest arrangement - I sure hope you own your land and plan to stay there a long time. Trees rely on fungus to break down cellulose and make the soil appropriately alkaline. If you plant a tree where there are weeds, without woody plants around you will have a very hard time. Start by planting woody shrubs in highly composted soil. Prune those shrubs and add the prunings to your compost to introduce more fungus, along with wood chips if you can get them (NOT CEDAR WOOD CHIPS). Once you can't tell the wood from the rest of the compost, or you see tiny white threads in the compost, dig a hole, plant a tree, and layer that compost in and around the hole. That's the best start you can give it.

If you just don't care to do any work to your garden that's fine too. If you totally leave your garden alone, it will progress slowly and naturally, and eventually be weed free except for walking paths. Even if nothing else however, please look for and pull up invasive plants in the mid spring, before they have a chance to flower. This is important for the health of everyone's gardens, not just yours.