I purchased a liquid laundry detergent that doesn't clean well at all and instead of throwing the bottle away, what are some alternative uses around the house for liquid laundry detergent?
Just kidding. Have you tried it as a pre-treatment for stains? That might not work either if it's terrible - even at full-strength. You didn't mention the brand/maker, which would make it less general. There are a few posters with strong Internet search prowess and others who understand chemicals better. Either approach may give you some ideas if they choose to respond.
Frankly, there is not much you can do with it if it won't clean well. Perhaps you could continue to use for laundry, just the lightly soiled loads that need a refresh rather than an heavy-duty wash.
Give it away to pet shelter.
Or use it as a qualified all-purpose cleaner for certain things. My particular fave is for washing cat litter boxes and pet carriers.
Occasionally I odds and ends of non-favorite detergent for initial scrubbing down of auction or CL finds. (I don't use detergent out of doors where it would stay on surface and perhaps wash into streams, only indoors in my utility sink.)