Can I say that I think you did very well? I think you're getting a very hard time for not bringing a gift and for not talking to the other mums. You were keeping an eye out for your DD so that she didn't have a melt down or created an issue at the party and I think that was your priority. Your DD was there to have fun.
The other mums/parents at the party could easily have come over to you to say hello and instigate the conversation, and it appears that they didn't.
For the next party, and I'm hoping that your DD will be invited to more parties as it does get easier, can I recommend the following?
Based on the age of your DD and only if you can afford it, get some sets of markers/colouring pencils/cheap waterpaint sets, a couple of colouring books and a few gift bags. Make up your own sets of gifts from these and just grab one gift bag as you're heading to the party. Another generic gift would be a skipping rope and some sweets or a reasonably priced jigsaw (not the 1000 piece ones, try for that age maybe 50/100 pieces). A board game like snakes & ladders or ludo would be something that a 5yr old could grow into playing with their family.
Go into the card gallery and get some of their 5 cards for £1 for kids birthdays or get your DD to make a card.
Keep an eye out for the sales on Amazon or in Smyths Toys or in Tesco for these types of things and you'll be golden!
If you are going to be staying, some small talk conversation topics would be the weather (we do really love going on and on and on about it 😁), things like Strictly Come Dancing or I'm A Celebrity (if you watch those), the latest releases on Netflix or what's about to come into the cinema. Then the topics can be about how your and their kids are getting on at school/nursery/college/whatever. I will say though, if these are parents that appear to be in a clique, it's very hard to break into an existing clique (I've found) but you can say hi and have a brief conversation. You are also perfectly allowed to leave the room for some fresh air (to gather yourself) in order to prepare for round 2 or whatever of the conversation. Circulate in the room. Chat to other parents. Offer to help bring food in from and out to the kitchen.
As for not dressing appropriately - ignore that one. That is just out and out rudeness.
I honestly don't think you did anything wrong. You could always send your DD in with a gift bag when they are going into school again and say that you were waiting until pay day and apologise for not having it on the day of the party. There are still ways to recover this, both for you and for your DD.