Getting stronger muscles doesn't require adding mass. Developing more strength through specific training may or may not benefit you (opinions on this are mixed; personally I think it probably depends on the rider). Adding more weight in the course of doing strength training might not hurt you if it turns out to have been a limiter, but it's very unlikely to benefit you. Despite the reputation of crit racers as being bigger and stronger, the very best crit racers in the country aren't, for the most part, 190 lbs or anything. They're still pretty slim. There are exceptions, but, well, they're the exceptions.
To the extent that bigger riders have an advantage on certain types of courses it comes down to two things. First, bigger people tend to have bigger lungs and bigger muscles, which means greater lung capacity, greater uptake of oxygen, greater absolute VO2max and therefore greater absolute power, all other things being equal (life isn't fair, so all other things generally aren't equal, but basically yeah, bigger people often make more watts). Second, bigger people have a lower surface area to volume ratio, which can more easily translate to a good watts:cda ratio relative to a smaller rider. But again, life isn't fair and it's more a correlation than a rule.
With respect to the first point, you can add more weight in the form of muscle, yes, but you're not very likely to change your lung capacity to match. So it's not like the heavier you get, the stronger you get. That's why adding weight is usually detrimental. With respect to the second point, same answer. Your overall size isn't going to change much, so adding weight isn't really going to help your aerodynamics and if anything will hurt them. If you get heavier AND faster, usually it means either you were underweight to begin with (extremely rare), or you've started training much more effectively at the same time (very plausible) and your gains from that training have outpaced the penalty of having more weight. Not that being heavier is helpful.