As well as it's other effects, alcohol tends to exaggerate your current mood, if you are feeling depressed you may feel more depressed, if you are feeling good, you may feel better. It can also reduce your ability to control your emotions, so for some autistic people the chance of having a meltdown could be higher when they drink. It does depend on the individual person however because alcohol effects different people in different ways and the typical causes of that person's meltdowns can be an important factor too. Many people are more relaxed after a drink and an autistic person may for instance feel more comfortable in a busy and noisy room than they would normally, if this is a particular issue that causes meltdowns for that individual, then alcohol could reduce the chances of having one.
I've never been an alcoholic because I've always been able to do without it, but like most adults I've had times where I've drunk far too much, I however don't drink alcohol at all any more and haven't done for over 15 years. When I did drink it relaxed me most of the time and gave me more confidence, although this wasn't always a good thing because it also put me in more danger and I'd be even more prone to upsetting NTs without realising it, but if I was feeling particularly depressed alcohol would only make me feel worse and would probably make me make a fool of myself. Overall however I've had good and bad experiences of alcohol, I've had a couple of meltdowns on alcohol, but it never really increased the risk for myself personally because the good and bad effects seemed to balance out overall.
For most people alcohol is okay in moderation, but obviously it can become very detrimental for some people, especially if they become physically addicted to it, if that happens to an autistic person the whole awful experience of being an alcoholic would in my opinion increase the chances of having meltdowns as the person generally goes downhill. Severe alcohol addiction can in some ways be even worse than heroin addiction because even though a heroin withdrawal is one of the worst experiences anyone could ever go through, it barely ever directly causes death even though the person often feels like it will, severe alcohol withdrawal can literally kill however and is extremely dangerous.