Is it safe to travel to Miami Beach shortly after hurricane Irma?

Miami Airport experienced "significant water damage" in the storm. It will be closed on Monday the 11th, and they'll determine tomorrow whether they're able to resume flights on the 12th or whether further repairs are necessary. Each airline will decide when to operate once the airport reopens.

Miami Beach was evacuated and will not allow people back in until Tuesday the 12th at noon (and then, only residents with ID) and some areas are flooded today. 75% of Miami-Dade County has no electricity right now, and the county is under a curfew. It is likely that your hotel doesn't know whether they can operate or whether they have power and won't know until after the roads reopen and staff can reach the hotel.

There's likely to be a lot more clarity by Wednesday, after they've started to clear the roads, people have had a chance to get back into Miami Beach, electrical crews have begun repairs, and the extent of the damage is surveyed. The city does "foresee" being open to visitors by Thursday, but you'll want to keep an eye on their communications and the Miami Herald newspaper for updates. That said, to me, Friday seems quite early to visit Miami Beach.

Personally, I'd change my trip to start someplace else in the US besides Florida on the 15th, keep an eye on the news, and go enjoy Miami starting on the 26th, assuming that things are sufficiently open then to have a good time.

This is not quite an answer to your question, but I think it might be useful due to your recent comment that you're booked on BA (instead of a submarine) and you did ask for suggestions:

https://www.britishairways.com/travel/flightops/public/en_gb

We are also providing rebook options for customers booked to fly with us to/from Miami, Fort Lauderdale, Tampa, or Orlando between now and 17 September inclusive. They can choose to fly to the same destination as originally booked up to and including 14 October. Customers can also use the value of their ticket towards an alternative destination on our network if they no longer wish to travel to/from Florida.

If you have not yet rebooked, call BA and ask for the cost of your existing flight to be applied towards your new one. If you have already rebooked without availing yourself of this, you have a 24 free cancellation period in order which could allow you to cancel and then rebook.

As mentioned, Irma will be gone, but the destructions caused by it will be visible for sure at that time.

Change your flight, if able.

A friend changes his flight from Athens to Miami, with only 100 euros extra charge, because he was afraid that he would be trapped to Constantinopole (transit airport for the flight). After two days the price for a date change went to 1200 euros.

I do not live in Miami, however I planned a trip for next month. First I would call the hotel to ensure it will be open and available for use.
But it also depends on the area in which you are traveling to, as Florida Keys received one of the biggest hits. I am following hashtags of Miami and Miami beach area and business owners and residents have returned as of September 12 2017.
A lot of the pictures and videos show trees that have fallen, however it appears as if the damage is being repaired for this, nothing showed major flooding and business are returning back to normal. As long as your hotel is up in running I think you will be fine, as the beach looks beautiful on the pictures.

Don't let the comments about Jose scare you, as the tropical storm is headed north east and is not headed in the direction of Miami.