That is a big mistake, investing when the bull run starts is a wrong move, you need to buy when the bear trend is on. Of course I would only buy bitcoin and not altcoins or the hyped projects that are running in the name of altcoins.
It depends, investing in bitcoin and other solid altcoins like ethereum or the binance coin when they are going down or they are in the middle of a bear market is without a doubt a good idea, however it is not a good idea to do so if you are interested in new altcoins, this is why if you are interested in taking such a risk and invest in those new coins then you have no other option but to wait until the bull market comes, as those coins tend to do well at that time, but then once the bull market begins to slow down they tend to crash at an alarming speed.