How could Sidious be more powerful than Yoda? Easy. Because the plot demanded it.
While certainly true, I see no reason for that not to be the case at any point. People assume Yoda is the best at everything, but there isn't proof of this.
Actually, when it comes to lightsaber battles, no one would expect him to be great. That's why his fight with Dooku was almost played for laughs in the second one (well, technically, his walking away with his cane after was the joke, but it illustrates the point that he was an old man(ish)).
They should've had Obi-Wan overpower Maul with his anger like Luke did to Vader in ROTJ.
Anyone notice in the original trilogy that the most effective way to win a lightsaber battle was to get really angry and start taking baseball swings at the other person? I personally think quite a lot of the fighting in the prequel was stupid (backflips), but I appreciate that it was actually possible to win through actual swordsmanship.
BTW, I wouldn't call the way Obi Wan defeated Maul to be poorly written, just poorly executed. They made it obvious and telegraphed for our benefit (almost in slow motion so you can appreciate what Obi Wan did), but it made Maul look like a complete idiot. Same with Palpatine in the fight in the Chancellor's office. The Jedis look like they just stand there to get stabbed when I'm pretty sure he was supposed to be displaying superior reflexes or something to catch them by surprise.