I think the designs have generally remained strong since Agito (especially the Undead), but the characterization and emphasis on the monsters sort of determines their effectiveness. If the plot is built around the monster, they tend to come across well, even if the design is extremely simple (this is the magic of the older Riders- low-budget costumes being treated as high-concept ideas, which in turn elevates the costumes because suddenly there's something behind the zippers and the visible stunt guys' eyes.)
If they're just dropped in as an excuse to get two Riders into a fight later or something to distract the hero for a bit from another plot line, they're going to be pretty forgettable no matter how good the design is. A couple of the post-Agito shows do fall into this trap, though I think it's a case-by-case basis. Faiz has some great, memorable monsters of the week that are genuinely creepy/weird. It also has Pigeon Orphenoch, and all I remember him for is being defeated by Kiba when he was Faiz, or Barnacle Orphenoch, who gets killed by Takumi that time he turns into Delta. Cool designs, but their most memorable qualities are who they were beaten by.
And I dunno, but I think the monsters should be more than that. One thing I'm glad we've seen in the post-Agito shows (or really, from the Agito movie on) is the concept of henchmen/henchmonsters slowly coming back, because those are the guys you can use for fodder. I liked that in Begins Night- rather than use it on a monster who had been unbeatable before, Fang Joker debuts against a bunch of thugs, but still comes off as impressive even if he's just frying the small fish.
Personally, I think that Kamen Rider should try to be scary within its limits. It should also try to be a lot of other things (uplifting, funny, melancholy, heroic, etc.) but providing some sense of fear is important. Scary monsters = Strong heroes, IMHO. If the villains are portrayed as frightening, powerful and a legitimate threat, it reflects well on the good guys. It kind of justifies all the gimmicks and power-ups because if they're up against a serious threat, they need all the help they can get. In fact, having threatening, frightening villains even kind of justifies the comedy. I'm okay with them clowning around in the off scenes because now I also really care about the people and what happens to them. It provides a huge tone shift, but that's the point.