Is it okay to use ex-dividend dates as an indication for shorting shares?

No, such a strategy wouldn’t work as the spread betting prices are adjusted for dividends. And to get a dividend (when buying real shares) you need to be onboard BEFORE it shows as ex-dividend. [XD day is usually (not always) a Wednesday, so you need to be onboard by close on the Tuesday. You can sell as early as you like after trading begins on the Wednesday and still qualify for the dividend which will come to you later).

Although you cannot (sbet) benefit from the price drop associated with the ex-dividend effect, there will be some stocks where existing shareholders were only hanging on for the dividend before quitting. In this respect some stocks can decline further or for longer than the expected drop – but it isn’t a good shorting strategy generally.